Dictionary:

antiphrasis

  (ăn-tĭf'rə-sĭs)

The use of a word or phrase in a sense contrary to its normal meaning for ironic or humorous effect, as in a mere babe of 40 years.

[Late Latin, from Greek, from antiphrazein, to express by the opposite : anti-, anti- + phrazein, to speak; see phrase.]

antiphrasis [an‐tif‐ră‐sis], a figure of speech in which a single word is used in a sense directly opposite to its usual meaning, as in the naming of a giant as ‘Tiny’ or of an enemy as ‘friend’; the briefest form of irony.

Adjective: antiphrastic.

The ironic or humorous use of words in a sense not in accord with their literal meaning, as in "a giant of three feet four inches."

The noun has one meaning:

Meaning #1: the use of a word in a sense opposite to its normal sense (especially in irony)

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