Inspired by a recent question about the proper way to write 生きがい, I wanted to ask a long-standing question.
According to Kotobank, 甲斐 has the following definitions (my poor translations follow):
- An old Japanese state, lying within modern Yamanashi.
- A city in western Yamanashi.
- When attached to the ren'youkei, expresses the effect, value or worth of the action.
- When attached to a noun, expresses that something commands a quality appropriate to that thing.
- When attached to a phrase that expresses a negation, expresses the extent to which it is fortunate that it is not so.
- When attached to a phrase that expresses a wish or desire, expresses the desire to do it as much as one pleases.
I do see the semantic relationship between the four senses of がい — all relate to a sense of worthiness or appropriateness — but why were these kanji, which relate to a place name of all things, applied to this concept?