Saying one thing while meaning another, often the opposite. The true sense is left to the reader or the listener.
In the Aeneid
You can take Aeneas out of the hands of the Greeks and put in front of him, in place of the man, a cloud and empty winds; you can turn the fleet into as many nymphs; and for us, to have helped the Rutulians in any way is a crime!
Juno ends with “is a crime!” said with sarcasm: she knows that helping the Rutulians is no worse than what Venus did for Aeneas. She says the opposite of what she thinks in order to accuse the unfairness between the two goddesses.