"The lady doth protest too much, methinks" is a line from the play Hamlet by William Shakespeare. It is spoken by Queen Gertrude in response to the insincere overacting of a character in the play within a play created by Prince Hamlet to elicit evidence of his uncle's guilt in the murder of his father, the King of Denmark.
The phrase is used in everyday speech to indicate doubt of someone's sincerity, and especially that someone who denies something very strongly is hiding the truth. In this sense the line is often misquoted as "Methinks the lady doth protest too much."