WordNet records no synonym and no antonym for slapstick. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a boisterous comedy with chases and collisions and practical jokes
acoustic device consisting of two paddles hinged together; used by an actor to make a loud noise without inflicting injury when striking someone
1 of the 3 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for slapstick.
Synonyms, antonyms and the broader and narrower relations for slapstick come from Princeton University’s WordNet. Full sources and licences.