IPA: /ˌæntɪˈfræsɪs/
KK: /æn'tɪfrəsɪs/
The use of a word or phrase in a way that means the opposite of its usual meaning, often for humor or irony.
Calling a slow runner 'Usain Bolt' is an example of antiphrasis.
Antiphrasis is formed from "anti-" (meaning against or opposite) and "phrasis" (from Greek "phrasis", meaning speech or expression). The term refers to a figure of speech where a word or phrase is used in a sense opposite to its normal meaning, often for ironic or humorous effect.
Think of 'anti-' meaning 'against' and 'phrasis' meaning 'speech' — this helps you remember that antiphrasis is when speech is used in an opposite way.