lie through teeth

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

Fig. to lie boldly. I knew she was lying through her teeth, but I didn't want to say so just then. If John denies it he's lying through his teeth, because I saw him do it.

To lie outrageously or brazenly.

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.