expression
it's tactless to state the obvious; that's best left unsaid
Used when pointing out something obvious would be crude or insensitive; often implies the listener already knows. A fixed expression.
彼が遅刻した理由は言うだけ野暮だよ。
It would be tactless to spell out why he was late.
あの二人の関係は言うだけ野暮だから、そっとしておこう。
Their relationship is best left unsaid, so let's leave it alone.