with (a) forked tongue
Duplicitously or beguilingly; untruthfully. It's becoming increasingly obvious that the president-elect spoke with forked tongue during his campaign. He learned quickly that speaking with a forked tongue was an effective way of getting ahead in business.
with forked tongue
untruthfully or deceitfully. humorous The image is of the forked tongue of a snake, snakes being traditional symbols of treachery and deceit.
2002 New York Times Orpheus members have long spoken with forked tongues about conductors. They…make sweeping generalizations about them.