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The word 同い年 is presumably related to the adjective 同じ. But what happened to
the consonant in the root? I thought this might be an example of イ音便, but
typically this happens to -k, -g only.

Is this odd change idiosyncratic to this word? Or does 同じ decompose further
into ona-j-i such that there is some other root ona-?
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2022-07-21T23:55:38.933
95478
2022-07-22T05:32:28.910
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question
etymology
What's going on in the word 同い年?
I had a look at the 日本国語大辞典【にほんこくごだいじてん】 (NKD)
[entry](https://kotobank.jp/word/%E5%90%8C%E5%B9%B4-581142). The NKD is a bit
like the Oxford English, it's one of the better dictionaries for giving word
derivations.

According to the entry, this reading _onaidoshi_ is an irregular sound shift
from older _onajitoshi_. The _onaidoshi_ version first shows up in 1563.

Phonologically, it looks like the voicing on the じ influenced the と to voice
to ど. And with the じ and the ど both starting with front-of-the-mouth
consonants, I can see how the consonant in じ would elide (become omitted).

→ At any rate, consider this particular word as a kind of one-off: memorize
it, but don't view this as indicative of any common pattern where ～じ becomes
～い.
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2022-07-22T05:21:35.930
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answer