buckle has 4 distinct synonyms spread across 4 of its 5 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. Its one recorded opposite is unbuckle.
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.
bend out of shape, as under pressure or from heat
“The highway buckled during the heat wave”
fastener that fastens together two ends of a belt or strap; often has loose prong
Synonyms, antonyms and the broader and narrower relations for buckle come from Princeton University’s WordNet. Full sources and licences.