Saying one thing while meaning another, often the opposite. The true meaning is left to the reader or listener.
In Purgatory
Now make yourselves proud, and go with a lofty face, sons of Eve, and do not lower your face, so that you do not see your evil path!
The poet orders the opposite of what he wants: "make yourselves proud, and go with a lofty face." Said to those who have just seen the floor full of falls, the apparent advice works as censure, and the cruel grace is to let the proud go with head held high.
Cantos IX to XII: The gate and pride, Dante, narrator, before the floor with the falls of the proud