antiphrasis[an tif′rə sis]
antiphrasis definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
the use of words or phrases in a sense opposite to the usual one, as for ironic effect
Origin: Classical Latin ; from Classical Greek ; from anti-, against plush phrazein, to speak
antiphrasis definition by American Heritage Dictionary
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.
Origin: Late Latin, from Greek, from antiphrazein, to express by the opposite : anti-, anti- + phrazein, to speak; see phrase.