Is the ’’おう’’' in the Volitional ending of verbs related to ''こう、そう、どう'' in any way?

Question

Is there any knowledge on where the volitional ''おう'' Ending originated? As far as my interpretations go having seen a lot of sentences with them, Both the ''おう'' of the volitional and the ''おう'' of ’’こう、そう、どう'' seem to indicate some kind of..estimate? Something uncertain/imprecise. Instead of being 100% precise it's just a kind of general direction, a kind of estimate or expectation that is probably that way but not 100%.

それ is certain, it is that, そう is more vague, it is ''that way''. And it is used for ''seems like'' hearsay. 行く　is just going. 行こう is more of an estimate, in that you'll ''probably'' go, or you say it when you say ''let's'' go, which isn't completely certain.

Another thing is how the other rows all start with the distance, and then some kind of thing that modifies it. like, その Is それ but possessive instead, so you use it in front of words.

Thing is, the こうrow has one that's ''ああ’’ which doesn't fall in line with this at all, but it may also be a sound change for all I know.

For all I know this could be a complete coincidence or my interpretation is completely false, soo, are they related in any way?

Answer 1

Nope, they're unrelated.

Although they look similar today, the う in こう comes from the く of 斯{か}く, and the う of 食べよう comes from the conjectural auxiliary む, so they were originally quite different.

• こう koʜ 'this way' comes from koʜ < kau < kaku. When kaku became kau, it caused a regular sound change from au to oʜ. With this change the word better fit the modern こそあど pattern phonologically, which allowed Japanese speakers to fill out the rest of the the こう・そう・ああ・どう koʜ soʜ aʜ doʜ paradigm by analogy, taking ʜ as a new morpheme meaning 'manner; way'.

  Note that the words ああ aʜ and どう doʜ are quite recent; these two words did not appear until Modern Japanese and were clearly formed by analogy. そう soʜ is slightly older, and Frellesvig gives soʜ < sau as a derivation, which goes back to Old Japanese sa(te).

  In terms of meaning, these four adverbs express 'manner; way'. I don't think the core idea of 'uncertain' or 'imprecise' is really included in the ʜ morpheme in these words.

• The suffix (y)oʜ, written with an オ段 kana ＋ う in Japanese orthography, comes from the Old Japanese conjectural inflecting auxiliary -(a)m. By Late Middle Japanese its conclusive form -(a)mu, written む in Japanese, had become -(a)u, and this caused the same regular sound change from au to oʜ.

  To avoid hiatus, the semivowel y is inserted between vowel-stem verbs and oʜ:

    行く　 ik-   + う (y)oʜ = 行こう　 ik-oʜ  
    食べる tabe- + う (y)oʜ = 食べよう tabe-yoʜ

  Although the range of meanings expressed by -(a)mu and now (y)oʜ has varied quite a bit, I think the core idea behind this auxiliary/suffix is expressing some kind of modality, ranging from epistemic to boulomaic. The former fits what you're expressing with 'uncertain' or 'imprecise', although it's been used to express a somewhat wider range of meanings than just epistemic modality.

Note that in this answer the symbol ʜ is used to indicate the long vowel phoneme. In each word there is a long vowel sound today and not an う u sound, but the words are nonetheless written with the kana う in Japanese orthography, even for words like どう doʜ which were never pronounced with u historically.