expression
asking would be rude; it's tasteless to ask
Used when the answer is obvious or the question itself is considered insensitive or boorish. Often said to gently chide someone for asking something they should already know or shouldn't ask.
「彼女、どうして泣いてるの?」「聞くだけ野暮だよ。」
"Why is she crying?" "It's rude to ask."
そんなことは聞くだけ野暮ってもんだ。
It's just tasteless to ask such a thing.