The Yamata no Orochi, as any fule kno, was a giant serpent slain in Japan's Heroic Age by Susonoo. "Orochi", though slightly etymologically murky, apparently means "[some kind of] serpent," and "Yamata" is generally understood (and, in the original sources, written) as "eight-forked" (八俣 or 八岐).

In the Kojiki, the Yamata no Orochi is described as follows.

(Incidentally, I'm following Saigō for Kojiki quotes throughout this post, w/r/t both kanji [which is all the original text has] and native Japanese "reading" [which is disputable and indeed disputed]. Readings are romanized a la Frellesvig. Translations are mine, indebted to Saigō's commentary.)

毎年來喫。 [...] 彼目如赤加賀智而、身一有八頭八尾。亦其身生蘿及檜榲、其長度谿八谷峽八尾而、見其腹者、悉常血爛也。

tosi goto ni kite kuraperi [...] sono me pa akakagati no gotoku site, mi pitotu ni ya kasira ya wo ari. mata sono mi ni koke to pisugi to opwi, sono take pa tani ya tani wo ya wo ni watarite, sono para wo mireba, kotogotoku ni tune ni ti ni tadaretari. (Saigō 2:191-193)

Every year it comes to feed. ... Its eyes are like winter cherries; its body has eight heads and eight tails, and is covered in moss, Japanese cypresses and Japanese cedars. Its length extends across eight valleys and eight hills, and if you look at its belly, it is always covered in blood.

In the Nihon Shoki (Sakamoto et al 1:92, 1:447):

頭尾各有八岐。眼如赤酸醤。赤酸醤、此云阿箇箇鵝知。松柏生於背上、而蔓延於八丘八谷之間。

Its head and tail both have eight forks. Its eyes are like winter cherries. Pines and Japanese cedars grow on its back, and it extends across eight hills and eight valleys.

Alexander Vovin's "Pre-Hankul Materials, Koreo-Japonic, and Altaic" raises an interesting point about these descriptions:

At first sight, there is an obvious contradiction: while the Kojiki describes the serpent as having eight heads and tails, the Nihonshoki tells us about eight forks: that is, the serpent should have nine heads and nine tails. More careful scrutiny of both texts, however, reveals that the serpent had eight and not nine heads: Susanowo tricks him by placing eight big jars with sake for each head, which the serpent drinks, then gets drunk, and is subsequently killed. [...] The only way out of this confusion is to assume that -mata in Ya-mata really means "head" and not "fork." (144)

This -mata, Professor Vovin argues, would in turn be related to *matay or *matæ, Vovin's reconstruction of a twelfth-century Korean word for "head" recorded in Chinese characters as 麻帝. The use of 俣 and 岐 (both meaning "fork" in some sense) in the Japanese texts instead of a character meaning "head" is "best explained as ateji," Vovin proposes. "It is also likely that by the early eighth century the real meaning of the word *-mata "head" in Ya-mata was already forgotten, and only the context of the myth in combination with simple arithmetic allows us to reconstruct it" (145).

This is an interesting and bold hypothesis, and Professor Vovin seems quite confident that it is true as he includes -mata in lists of Old Japanese loans from Korea in both Vovin 2005 and Vovin 2010 (in both cases with the note that yamata no orochi is the only example of this mata.)

However, with all due respect to Professor Vovin, I do not feel that this theory is supported by the evidence. X-mata does not mean "forked X times (into X+1 child nodes)." It means "forked into X child nodes" — and mutatis mutandis for related constructions like the Nihon shoki's 有八岐 "had eight forks". There is therefore no conflict between the Yamata no Orochi's name and head-valency, and no need to invoke a nonce-borrowing from Korea.

For example, consider the word putamata. This word is morphologically identical to yamata except with "two" (puta) instead of "eight" (ya). Conveniently, it also appears in the Kojiki, in the Emperor Suinin chapter:

[...] 二俣榲。作二俣小舟而。持上來以。浮倭之市師池。輕池。[...]

[...] putamata-sugi wo putamata-wobune ni tukurite, moti noborikite, Yamato no Itisi no Ike, Karu no Ike ni ukabete [...] (Saigō 5:316-317)

[... They] made the two-forked Japanese Cedar into a two-forked boat, carrying it up to Yamato and floating it on Ichishi Pond and Karu Pond there [...]

This episode is no doubt related to similar stories in the Nihon Shoki: in the Emperor Nintoku chapter, a tree described as "本一以末両" ("one trunk [forked] double") floats down the river and is made into a boat; in year 3 of Emperor Richū's reign, the Emperor launches a "double-forked boat" (両枝船) on Ichishi Pond (市磯池) in Iware (磐余), with the note that he sat on one side and his consort on the other (與皇妃各分乘而遊宴) (Saigō 5:317-318).

There is no argument, as far as I am aware, that these trees and boats have three or four ends (lobes, hulls, whatever) protruding from two separate instances of forking. They are understood to be an object that is united at its base but forked, once, into two.

(Sakamoto et al connect the "double-forked boat" motif of the Nihon Shoki to Southeast Asian/Polynesian catamaran technology (Sakamoto et al 2:449). There is also an interesting discussion of myths and taboos surrounding forked trees in Tsunemitsu, including the legend, new to me, that Kakinomoto no Hitomaru was born from a fork in a persimmon tree (Tsunemitsu 442); the inevitable connection of the forked tree with the manner in which humans are born (431, quoting Hotta); and the observation that in the Kojiki itself Ōkuninushi escaped/was reborn via the fork of a tree after being killed by his eighty brothers for the second time (441).)

We can find more examples by going forward in time. The 1603 Vocabvlario da lingoa de Iapam offers us:

• Futamata. Forquilha de duas pontas. (Vocabvlario 112)
• Mitçumata. Forquilha de tres pontas. (363)

In the Sino-Japanese department of the lexicon, we have the morphologically equivalent sansa 三叉, the Nihon kokugo daijiten's earliest citation for which is from the 1508 Rokumotsu zushō 六物図抄 and clearly describes a trident-like, or rather forward-facing-part-of-a-chicken-claw like (same diff, sorry Poseidon) structure: "鳥の爪を前へ三つかけてつかむが如し". Similarly, trigeminal nerve is known as the 三叉神経, with the 1872 Igo ruiji 医語類聚 ("Anthology of medical terminology") the NKD's earliest citation. (I bring this one up because it illustrates that Indo-Europe can use "fork"-like morphemes in the same way: as I understand it, etymologically, "trigeminal" doesn't mean "thrice-twinned [for a total of four]", it means "'twinned' into three.")

Incidentally, futamata is still a word in modern Japanese, and one of the metaphorical senses it has picked up over the centuries is "cheating [on a partner in a romantic relationship]." The folk etymology for this sense is, ahem, "two-crotched," but of course the real etymologically is "divided into two," what we might call "two-faced" in English.

It seems to me that the general meaning of X-mata and equivalent Sino-Japanese constructions, has been from antiquity and still remains "forked into X child nodes," not "forked X times into X+1 child nodes." I see no reason to assume that yamata works differently, except insofar as the use of "eight" also hints at "countless." Thus, using the word yamata to describe an eight-headed dragon is not problematic, and there is no need to invoke an otherwise unattested loanword from Korea.

Why "forks" and not "heads"?

You might ask, well, if this is the case, why use "fork" at all? Why not just say "eight-headed" and be done with it? This is a reasonable question, and I imagine the answer has to do with the Yamata no Orochi as a symbol of Japan's then-untamable and terrifying rivers (see e.g. Tsugita 1:103-104 for discussion and comparison with other mythologies; see also Juhl for a similar story from eastern Japan in the Enoshima Engi).

Rivers, like roads, were more naturally understood as "forked" than "headed". In the "Dialects" section of its entry for futamata, the NKD lists "point where rivers meet" (川の合流点) as a Nagano-specific meaning, and "valley divided into two" (二つに別れている谷) for Oita prefecture. Note also that the Yamata Orochi has eight heads and eight tails; being "split into eight [=many]" is clearly more basic to its nature than simple eight-headedness. So it doesn't really seem odd to me that a "fork" rather than a "head" morpheme would find its way into the name of a river symbol.

References

• Frellesvig, Bjarke. A History of the Japanese Language. Cambridge: Cambridge University Press, 2010. Print.
• Hotta, Yoshio 堀田吉雄. Yama no Kami Shinkō no Kenkyū 山と神信仰の研究. Kuwana: Ise Minzoku Gakkai 伊勢民族学会, 1966. Print.
• Juhl, Robert. The Goddess, the Dragon, and the Island. 2003. Web. 5 March 2012.
• Nihon Kokugo Daijiten 日本国語大辞典. Shōgakukan 小学館. JapanKnowledge. Web. 5 March 2012.
• Sakamoto, Tarō 坂本太郎, et al, eds. Nihon Shoki 日本書紀. 1994. 5 vols. Tokyo: Iwanami Shoten 岩波書店, 2001. Print.
• Saigō, Nobutsuna 西郷信綱. Kojiki Chūshaku 古事記注釈. 8 vols. Tokyo: Chikuma Shobō 筑摩書房, 2005. Print.
• Tsugita, Masaki, ed. Kojiki 古事記. 1977. 3 vols. Tokyo: Kōdansha 講談社, 1993. Print.
• Tsunemitsu, Tōru 常光徹. "Futamata no Reisei to Kaii Denshō" 二股の霊性と怪異伝承. Nihon kaibutsugaku taizen 日本妖怪学大全. Ed. Komatsu Kazuhiko 小松和彦. Tokyo, Shōgakukan 小学館: 2003. 425-443. Print.
• Vocabvlario da Lingoa de Iapam. Nagasaki, 1603. Tokyo: Benseisha 勉誠社, 1978. Print.
• Vovin, Alexander. "Pre-Hankul Materials, Koreo-Japonic, and Altaic." Korean Studies 24 (2000): 142-155. Monumenta Altaica. Web. 5 March 2012.
• ---. A Descriptive and Comparative Grammar of Western Old Japanese: Part 1: Phonology, Script, Lexicon and Nominals. Folkestone, Kent: Global Oriental, 2005. Print.
• ---. Koreo-Japonica: a re-evaluation of a common genetic origin. Honolulu: University of Hawaii Press, 2010. Print.