"frame with bars," c.1300, possibly from Middle Dutch rec "framework," literally "something stretched out, related to recken (modern rekken) "stretch out," cognate with Old English reccan "to stretch out," from Proto-Germanic *rak- (cognates: Old Saxon rekkian, Old Frisian reza, Old Norse rekja, Old High German recchen, German recken, Gothic uf-rakjan "to stretch out"), from PIE *rog-, from root *reg- "to move in a straight line" (see regal).Meaning "instrument of torture" first recorded early 15c., perhaps from German rackbank, originally an implement for stretching leather, etc. Mechanical meaning "toothed bar" is from 1797 (see pinion). Meaning "set of antlers" is first attested 1945, American English; hence slang sense of "a woman's breasts" (especially if large), by 1991. Meaning "framework for displaying clothes" is from 1948; hence off the rack (1951) of clothing, as opposed to tailored.

type of gait of a horse, 1580s, from rack (v.) "move with a fast, lively gait" 1520s in this sense (implied in racking), of unknown origin; perhaps from French racquassure "racking of a horse in his pace," itself of unknown origin. Or perhaps a variant of rock, v.1.

"clouds driven before the wind," c.1300, also "rush of wind, collision, crash," originally a northern word, possibly from Old English racu "cloud" (or an unrecorded Scandinavian cognate of it), reinforced by Old Norse rek "wreckage, jetsam," or by influence of Old English wræc "something driven;" from Proto-Germanic *wrakaz, from PIE root *wreg- "to push, shove, drive" (see urge, v.). Often confused with wrack, n., especially in phrase rack and ruin (1590s). The distinction is that rack is "driven clouds;" wrack is "seaweed cast up on shore."

"cut of animal meat and bones," 1560s, of unknown origin; perhaps from some resemblance to rack, n.1. Compare rack-bone "vertebrae" (1610s).