lie through one's teeth

To lie brazenly and unabashedly. Stop lying through your teeth—we have evidence that you were here the night of the crime.

Also, lie in one's teeth. Utter outrageous falsehoods, as in He was lying through his teeth when he said he'd never seen her before; they've known each other for years . This expression presumably alludes to a particular facial grimace one assumes when lying. [c. 1300]

To prevaricate outrageously. Versions of this seemingly modern expression appeared as long ago as the fourteenth century. William Safire cites its use in The Romance of Sir Guy of Warwick (“Thou lexst amidward thi teth”), as well as in a still earlier Northumbrian poem, but points out that Shakespeare preferred the throat to the teeth (Twelfth Night, 3.4; Hamlet, 2.2). Of more recent provenance is to lie like a trooper, dating from the late 1800s; the British version is to swear like a trooper. Why a trooper should have been singled out is a matter of conjecture. Presumably it alludes to the legendary lack of truthfulness in the military, especially the lower ranks, who lie to escape punishment. Originally “like a trooper” meant vigorously, or with great enthusiasm, which clearly was carried over to lying.