酸い

Etymology
From + adjective-forming suffix, modern. Appears as early as the late, in the 今昔物語集 (Konjaku Monogatari Shū, “Anthology of Tales from Today and Yesterday”).

The spelling with the kanji is based on synonyms in Chinese:, , and  all have senses related to , and all three can be used in Japanese to spell.

Adjective

 * 1) vinegary, sour