antigram
adj. An anagram in which the new word or phrase is the opposite of the original.

Example Citation:
The converse of the aptagram is the antigram. In antigrams, a word or phrase gets rejuggled into another word or phrase that bears a meaning opposite to that of the base. Thus, astronomers/moon starers is an aptagram while astonomers/no more stars is an antigram.
—Richard Lederer, "Looking at language; Check out this ever-changing parade of antigrams," The Patriot-Ledger, April 12, 1997

Earliest Citation:
Antigrams are anagrams that mean the opposite of the original word or phrase when the letters are rearranged. For example: violence — nice, love; evangelist — evil's agent; funeral — real fun.
—Marty Smith, "What's in a name," The Advertiser, July 8, 1991

Related Word:

Category: