na-adjective, noun
Describes someone, especially a younger or lower-status person, acting in a way that is irritatingly bold or disrespectful. Often used in the phrase 小癪なことを言う (say something cheeky).
あの子は小癪な口をきく。
That kid talks back cheekily.
小癪な真似をするんじゃない。
Don't get impudent with me.
生意気 is more common and direct for 'cheeky' or 'impudent', while 小癪 carries a slightly old-fashioned or literary feel and often implies irritation at someone acting above their station.
図々しい emphasizes shamelessness or thick-skinned behavior, whereas 小癪 focuses on irritating boldness or disrespect, often from someone younger.
The word is a compound of 小 (small, petty) and 癪 (irritation, a fit of anger). The exact historical derivation is uncertain, but it likely developed from the idea of something small that causes irritation, extending to describe impudent behavior.