unpleasant, disagreeable The word opposite pleasant is "unpleasant" Unpleasant, bothersome, worrisome, disagreeable, unhappy, hateful, nasty, unfriendly, troubling, unacceptable...
The opposite (antonym) of antonym is synonym. An antonym is a word that means the opposite. A synonym is a word that means the same thing, or nearly the same thing, for a given context.