enantiosemy
A linguistic phenomenon whereby a word constitutes its own antonym depending on context. English, being widely spoken and/or occasionally mangled in the United States and recalcitrant parts of the commonwealth (i.e., Canada), features many enantiosemic words, including inflammable, nonplussed, and restive (with the latter meaning "impatient and fidgety" when applied to people and, owing to its French roots, "stubbornly refusing to move" when applied to horses).
From Gr. enantios ("opposite") + semasia ("meaning").
From Gr. enantios ("opposite") + semasia ("meaning").
Gord: HOW DO MOUNTIES MANAGE TO BE NONPLUSSED ALL THE TIME, ANYWAY? THERE ARE GUYS WITH RACCOONS IN THEIR JACKETS AND STUFF—
Nigel: Actually, its prefix notwithstanding, and despite plussed's non-status as a word, "nonplussed" means the opposite of what it's commonly thought to mean.
Gord, nonplussed: OKAY, SO WE'RE TALKING ENANTIOSEMY, BUT LOOK, THE HORSE IS RESTIVE, AREN'T YA, BUD?
RCMP Thoroughbred: <snorts and offers an impregnable stare>
Inflammable means flammable? What a country! What enantiosemy!
―Dr. Nick Riviera, whose doctorate is in linguistics, not medicine
Nigel: Actually, its prefix notwithstanding, and despite plussed's non-status as a word, "nonplussed" means the opposite of what it's commonly thought to mean.
Gord, nonplussed: OKAY, SO WE'RE TALKING ENANTIOSEMY, BUT LOOK, THE HORSE IS RESTIVE, AREN'T YA, BUD?
RCMP Thoroughbred: <snorts and offers an impregnable stare>
Inflammable means flammable? What a country! What enantiosemy!
―Dr. Nick Riviera, whose doctorate is in linguistics, not medicine