あく

Etymology 1
Likely derived from verb, older form.

First cited to the of the early 900s, where the term is used as a pun for the verb.

The senses related to and  arose from the practice of using lye in food processing to remove unpleasant flavors.

The kanji spelling is an example of jukujikun, using the characters for  and  in reference to the practice of leaching wood ash in water to derive lye.

Noun

 * 1)  lye, originally the clear alkaline liquid left after leaching wood ash in water
 * [Note: This aku is also a pun on the verb .]
 * 1) scum, the coagulated proteins and fats that gather on the surface of a pot of boiling food
 * 2)  components contained in vegetables that impart an acrid, astringent, or harsh flavor
 * 3)    harshness as a characteristic of one's personality
 * 1)  components contained in vegetables that impart an acrid, astringent, or harsh flavor
 * 2)    harshness as a characteristic of one's personality
 * 1)    harshness as a characteristic of one's personality

Etymology 2
Various kanji readings.