antiphrasis (n.)

in rhetoric, "the use of a word in a sense opposite to its proper meaning; ironic use of a word in sarcasm or humor," 1530s, from Latin antiphrasis, from Greek antiphrasis, from antiphrazein "to express (something) by the opposite," from anti "against, opposite, instead of" (see anti-) + phrazein "to tell, declare, point out, express" (see phrase (n.)). Related: Antiphrastic.