Sue no chigiri is an 18th-19th century piece of Japanese music, originally composed by Matsuura Kengyō for shamisen and voice (a jiuta piece) but later arranged by Yaezaki Kengyō for an ensemble that also included the koto. Tsuge Gen'ichi translates the title "Pledge of Eternal Fidelity"; Ingrid Seldin and Okamoto Koji prefer "Vow of Eternal Love" (an entirely different proposition, really). The title is from the last couple of lines:

... Yachiyo furu tomo/ kimi mashite/ kokoro no sue no/ chigiri tagau na
... though I may live a thousand years/ so long may you,/ my beloved,/ not break our vow of eternal love! (Seldin and Okamoto, in Burnett 1984: 46-47)
... [Please do not forget me,]/ Even though I live [...] for eight thousand years/ As your mistress./ Do not break our sincere pledge/ Of eternal fidelity. (Tsuge 1983: 95)

But how do we get "eternal" from sue? Well...

Etymologically, sue means "end" or "tip." It is the opposite of moto, "base." Song 97 in the Nihon shoki:

komoriku no/ patuse no kapa yu
nagarekuru/ take no/ i-kumidake yodake
moto pye woba/ koto ni tukuri
suwe pye woba/ puye ni tukuri... (OJ romaji from Frellesvig et al 2013)

Down the river/ of Hatsuse the hidden land/ Comes floating/ A bamboo, /Interlaced bamboo, young bamboo:/ From the trunk/ They fashion a cithern,/ From the tip/ They fashion a flute... (Cranston 1993: 108)

Incidentally, in Iwanami's old NKBT edition of the Nihon shoki kayō, editor Tsuchihashi Yutaka 土橋寛 takes great pains to demolish the notion that the ancient Japanese might have literally made their koto ("citherns") out of bamboo; the couplet, he says, is there for structural and rhythmic purposes, and one should not seek meaning in each and every word (Tsuchihashi 1957: 212).

Anyway, it also developed metaphorically: the end of a path, the end of a period of time, things to come, one's children... The Nihon Kokugo Daijiten has a citation from the Pillow Book chigiri that also includes sue:

をとこ、女をばいはじ、女どちも、契りふかくてかたらふ人の、末までなかよき人かたし

When people, whether they be men or women or priests, have promised each other eternal friendship, it is rare for them to stay on good terms until the end. (Shōnagon 1971: 83)

(Where did he get "priests" from?)

Phrasing like sue made implies, to me at least, that sue can't in and of itself be eternity: it's a definite point, however far off. Even expressions like sue nagaku seem to imply a possibility of measurement that eternity does not.

The NKD also has citations for kokoro no sue, which it defines as "where the heart comes to rest; the future state of the heart" (心が移り行く先。将来の心の状態). So while "vow of eternal X" is fine as an idiomatic English translation, structurally sue no chigiri seems to mean something like "vow to end up doing X" — not a promise of passive faithfulness to last forever, but a vow to actively do something, and make sure it stays done, when the time is right — in the case of this poem, to rescue the narrator, implicitly some sort of courtesan, from her life of uncertainty and woe. For this reason, I think I prefer "Pledge of Eternal Fidelity" to "Vow of Eternal Love." The addressee here isn't just being asked to stay in love forever; he's being reminded that he gave his word to do something concrete, and that the narrator expects it to eventually get done.

Works cited

• Cranston, Edwin A. A Waka Anthology: Volume One: The Gem-Glistening Cup. Stanford: Stanford University Press, 1993.
• Frellesvig, Bjarke; Horn, Stephen Wright; Russell, Kerri L; and Sells, Peter. The Oxford Corpus of Old Japanese. 2013. <http://vsarpj.orinst.ox.ac.uk/corpus/>.
• Shōnagon, Sei. The Pillow Book of Sei Shōnagon. Ed. and Trans. Ivan Morris. London: Penguin, 1971.
• Tsuchihashi, Yutaka 土橋寛 and Konishi Jin'ichi 小西甚一, eds. Kodai kayōshū 古代歌謡集. Tokyo: Iwanami Shoten (Nihon Koten Bungaku Taikei 3), 1957.
• Tsuge, Gen'ichi. Anthology of sōkyoku and jiuta song texts. Tokyo: Academia Music, 1983.
• Burnett, Henry. "Anthology of Jiuta Poetry." Hogaku 1(2) (1984): 28-54.