Commenting on the anti-Buddha tanka quoted earlier this week, John asks:

Apparently うそ used to be written をそ. Is there anything interesting there? Is it the pronunciation of を that changed or the pronunciation of うそ?

This is a complicated question and I should warn everybody in advance that I don't have an actual answer.

First, a note: /uso/ is recorded centuries before Atsutane's time, in sources like the Kanginshū and the early 17th-C. Portuguese-Japanese dictionary Vocabvlario da Lingoa de Iapam. Atsutane's /woso/ is archaic at best.

That said... the pronunciation of を (originally /wo/) has changed, but it changed to /o/. /otoko/ (man) used to be /wotoko/, the MJ object-marking particle is pronounced /o/ but written を, etc. etc. And this change was entirely regular, so /woso/ → /uso/ would be an exception requiring an exceptional explanation. It turns out that this is not easy to find, and there may not be a direct connection between /woso/ and /uso/ at all.

In Norinaga's tanka, the key bit is not just /woso/ but the entire word /wosogoto/ (lies). This did survive into modern Japanese, in the expected form of /osogoto/, although I think it's a bit archaic now. So let's say the trail begins there.

Now, as it happens, the Kōjien definition includes a quotation from the Heian-period Ōgishō which is of interest:

東の国の者は、そらごとをば【おそごと】と言ふなり

The people of the eastern states call soragoto [lies] "osogoto".

So the /(w)oso/ → /uso/ theory would clearly involve this word coming from the east (and being abbreviated) and replacing the standard Heian /soragoto/ ("empty words"; the prefix /sora/ here is closely related [phonemically identical to, in fact] to the /sora/ that still means "sky" today). Interestingly, there are certain west:east::/u/:/o/ correspondences in very old Japanese sources. And you can see how /wo/ would become /u/ in the absence of a /wu/ mora. I think that eastern dialect thing is supposed to have faded out even before the Heian period, though, so I don't know how viable this idea is...

(From here)

... But, with the eastern connection in mind, another point of interest is a certain 東歌 ("eastern poem") in the Manyōshū which resembles Norinaga's tanka suspiciously closely:

可良須等布 於保乎曽杼里能 麻左Ｒ*尓毛 伎麻左奴伎美乎 許呂久等曽奈久

karasu tohu/ oho-woso-tori no/ masade ni mo/ kimasanu kimi wo/ "koroku" to zo naku

The oho-woso bird called the crow, though you have not really come, cries koroku!

Explanations:

• /koroku/ -- Most modern commentators, including ITŌ Haku, seem to agree that /koroku/, while obviously imitating a crow call, can be interpreted as koro ku, "自来", "[someone] themself arrives". (Some slightly less modern commentators, including the Kōjien, identify the /ko/ with the /ko/ that means child, but ŌNO Susumu points out that it is not the right kind of /-o/.)
• /oho-woso/ -- /oho/ is MJ /o:/, "big" or "great[ly]", and the /woso/ is generally glossed as "hasty". (Word on the streets is that it's related to the /wase/ meaning "early shoots" that you see in "Waseda" (早稲田), as in the university.)

... So the poem becomes one about an overeager crow announcing the beloved's arrival before they actually arrive. On the face of it, that doesn't give us any connection to /wosogoto/ or /uso/.

But, you know, people have changed their mind about what /ohowoso/ meant before. (Kigin thought it meant "eats a lot", from the OJ verb /wosu/, to eat.) Plus, it doesn't seem impossible that a word might wander from meaning "ill-considered, rash" to "inaccurate, untrue". Doubleplus, "bird which tells big fat lies" would work just as well if not better than "overly eager bird" in this context...

In any case, whether Norinaga's tanka is a direct reference to this one in the Manyōshū or not, he clearly identifies /woso/ as a word or at least a prefix meaning "lie", and attaches it to /-bito/ (person). (It doesn't make any sense to call Buddha "hasty" or "overeager", and the falsity of Buddhism is a repeated theme throughout the book.)

So you might conclude that the idea of /woso/ growing up to become /uso/, possibly after an incubation far from the capital in the eastern lands, is not entirely ridiculous.

Unfortunately, I have been saving a big complication: there was already an /uso/ in OJ. It meant "pursing your lips and blowing", "whistling", and this too survived into MJ (picking up a +archaic along the way).

Modern /uso/ meaning "lie" could clearly be a metaphorical derivative. /wosogoto/ could be, too, with some phoneme change thrown in. But it seems we don't have enough evidence to be sure exactly what went down.

Just to complicate things further, here are two bonus, totally unrelated proposed etymologies for /uso/, courtesy of the Nihon Gogen Daijiten:

1. Related to the OJ verb /usu/ (to disappear, fade out, die)
2. /so/ derived from /sora/, /u/ related to /uku/ (float; by extension, be worthless or silly.)

In conclusion: it's clear that Atsutane was using /woso/ to mean /uso/, but he may just have been mimicking a (much) older source, and the actual linguistic connection, if any, between /woso/ and /uso/ remains un-pinned down.