Definition
To insist so passionately about something that people suspect the opposite of what one is saying.
Origins
Shortened from the lady doth protest too much.
How People Actually Use It
Overly insistent about something, to the point where the opposite is most likely true.
"Someone emphatically stating how much one hates gossiping, that one would never engage in this sort of behavior. This implies that, "thou doth protest too much" and may in fact be a gossip."
Source: Urban Dictionary