the lady doth protest too much

Meanings

1. It is suspected that, because someone is insisting too much about something, the opposite of what they're saying must be true.

Etymology

An allusion to Shakespeare's Hamlet, where the line is spoken by Queen Gertrude, Hamlet's mother. In the play, "protest" is used to mean "insist that what one is saying is true" (in this case, the Player Queen's protestations of love), not "insist that what another is saying is false."