also: しりがあおい · けつがあおい
expression, i-adjective
wet behind the ears; inexperienced; immature
Idiomatic expression literally meaning 'butt is blue', from the bluish tinge of children's buttocks. Used to describe someone who is inexperienced, immature, or still a novice.
まだケツが青い新人に、そんな大事な仕事は任せられない。
We can't entrust such an important job to a newbie who's still wet behind the ears.
彼はケツが青いけど、やる気はある。
He's still green, but he's motivated.
未熟 is a more formal, neutral term for 'immature' or 'unskilled', while ケツが青い is a colloquial, idiomatic expression with a slightly derogatory or teasing nuance.
青二才 is a noun meaning 'greenhorn' or 'young upstart', often used dismissively. ケツが青い is an adjective phrase describing the state of being inexperienced.
From the bluish tinge of children's buttocks, implying youth and inexperience. The exact historical derivation is uncertain, but the phrase is a well-established idiom.