The lady doth protest too much, methinks
As attested in many an online source (and even some offline), Hamlet III.ii, spoken by Gertrude during the play within the play, which Hamlet is using to reenact how he thinks his father was murdered.
The Gertrude-within-the-play is swearing how much she loves her husband; Gertrude outside the play says the actress’ oaths are too effusive (protest = vow). Hamlet, who thinks Gertrude is complicit in Old Hamlet’s murder, responds “O, but she’ll keep her word.”