rack has 12 distinct synonyms spread across 8 of its 18 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.
stretch to the limits
“rack one's brains”
put on a rack and pinion
“rack a camera”
seize together, as of parallel ropes of a tackle in order to prevent running through the block
Synonyms, antonyms and the broader and narrower relations for rack come from Princeton University’s WordNet. Full sources and licences.
rib section of a forequarter of veal or pork or especially lamb or mutton
an instrument of torture that stretches or disjoints or mutilates victims
a support for displaying various articles
“the newspapers were arranged on a rack”
a form of torture in which pain is inflicted by stretching the body
a rapid gait of a horse in which each foot strikes the ground separately