WordNet records no synonym and no antonym for sidecar. 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 cocktail made of orange liqueur with lemon juice and brandy
conveyance consisting of a small carrier attached to the side of a motorcycle
Synonyms, antonyms and the broader and narrower relations for sidecar come from Princeton University’s WordNet. Full sources and licences.