sideboard has 2 distinct synonyms spread across 1 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 removable board fitted on the side of a wagon to increase its capacity
a board that forms part of the side of a bed or crib
a piece of furniture that stands at the side of a dining room; has shelves and drawers
Synonyms, antonyms and the broader and narrower relations for sideboard come from Princeton University’s WordNet. Full sources and licences.