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.
It would be tactless to spell out why he was late.
Their relationship is best left unsaid, so let's leave it alone.
A set phrase combining 言う (to say), だけ (only/just), and 野暮 (boorish/tactless). The exact origin is uncertain, but it reflects the idea that verbalizing something obvious is unsophisticated.