WordNet records no synonym and no antonym for cantilever. 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.
construct with girders and beams such that only one end is fixed
“Frank Lloyd Wright liked to cantilever his buildings”
Synonyms, antonyms and the broader and narrower relations for cantilever come from Princeton University’s WordNet. Full sources and licences.
projecting horizontal beam fixed at one end only