WordNet records no synonym and no antonym for bumper. 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 glass filled to the brim (especially as a toast)
“we quaffed a bumper of ale”
a mechanical device consisting of bars at either end of a vehicle to absorb shock and prevent serious damage
Synonyms, antonyms and the broader and narrower relations for bumper come from Princeton University’s WordNet. Full sources and licences.