literally, an excuse not asked for is a manifest accusation (against oneself). A bit of lawyerly Latin used when someone preemptively offers unnecessary excuses.

Or, as William Shakespeare expressed the concept (Hamlet, Act III, Scene 2):

Hamlet: Madam, how like you this play?
Queen Gertrude: The lady protests too much, methinks.

In the play, Hamlet has set up a play-within-a-play (called The Mouse-Trap, appropriately), to point an accusing finger at his mother, and to trap her and her new husband, King Claudius, into an admission that they connived at the murder of Hamlet's father.

Not realising that the queen in the play, who offers constant and excessive protestations of love and devotion for her dead husband, is meant to be herself, Queen Gertrude correctly points out that "the lady doth protest too much".