Saying one thing while meaning another, often the opposite. The true meaning is left to the reader or the listener.
In Othello
O, you are well tuned now! / But I'll set down the pegs that make this music, / As honest as I am.
Iago says the couple's melody is "as honest as I am" and means the opposite. Whoever heard the "I am not what I am" of Act I knows he is not honest, and the mockery announces that he will undo the harmony he has just heard.