Why 浴衣 is pronounced ゆかた, and why pronunciation ゆ and かた for kanji 浴 and 衣 are not listed in 国語辞書?

Question

浴衣{ゆかた} is a very common word. Why is it pronounced ゆかた, and why the corresponding pronunciations for the kanji 浴 and 衣 are not listed in 国語辞書? Pronunciation of 浴 「yù」 is mentioned only in 中日辞書, but I cannot find pronunciation 「かた」 for 衣 explained anywhere.

Answer 1

Well, it's ateji. 浴 (bathing) + 衣 (clothes) = 浴衣 (bathing clothes).

ゆかた (the pronunciation itself) is actually an abbreviation for the word 湯帷子{ゆかたびら}, which of course is a compound of 湯{ゆ} + 帷子{かたびら}. Etymologically, 帷子 is from 片{かた} + 枚{ひら} (a cognate of 平{ひら}), but its kanji too are ateji.

A 帷子 was an unlined bast fiber worn by the rich in the summer. 湯帷子 was from the Heian era and was used to dry off the body, since bath towels weren't used in Japan at the time. By the Edo period, cotton became preferred over hemp and ramie (the original bast fibers used) because it was softer, more absorbent, and was abundant. It was at this point that ゆかたびら was abbreviated to ゆかた and was given its new ateji characters, 浴衣. Nowadays they are used for more than just bathing, but that was their original purpose.

Sources: 1, 2, 3, 4, 5

Answer 2

It seems 浴衣 is a kind of 当て字 and originally was a short of 湯帷子(yukatabira). It's also read as よくい(yokui) when it's read by 音読み(on). But I doubt this reading is used except for the kinds of bathrobe today.