What does the phrase “to lie in one’s teeth” mean and Where does it come from?

To accuse a person of lying in his teeth is the strongest of accusations, implying that the person is such a double-dyed liar as to be unfamiliar with truth.

It is very old, traceable to the early 1300s, as in The Romances of Sir Guy of Warwick, “Thou liest amidward and therefore have thou maugreth (shown ill will).”