buck has 18 distinct synonyms spread across 7 of its 9 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 gymnastic horse without pommels and with one end elongated; used lengthwise for vaulting
a piece of paper money worth one dollar
United States author whose novels drew on her experiences as a missionary in China (1892-1973)
to strive with determination
“John is bucking for a promotion”
Synonyms, antonyms and the broader and narrower relations for buck come from Princeton University’s WordNet. Full sources and licences.