rack (n.1) Look up rack at Dictionary.com
"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.
rack (n.2) Look up rack at Dictionary.com
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).
rack (n.3) Look up rack at Dictionary.com
"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."
rack (v.) Look up rack at Dictionary.com
"to stretch out for drying," also "to torture on the rack," early 15c., from rack (n.1). Of other pains from 1580s. Figurative sense of "to torment" is from c.1600. Meaning "raise above a fair level" (of rent, etc.) is from 1550s. Meaning "fit with racks" is from 1580s. Teenager slang meaning "to sleep" is from 1960s (rack (n.) was Navy slang for "bed" in 1940s). Related: Racked; racking. Rack up "register, accumulate, achieve" is first attested 1943 (in "Billboard"), probably from method of keeping score in pool halls.
rack (n.4) Look up rack at Dictionary.com
"cut of animal meat and bones," 1560s, of unknown origin; perhaps from some resemblance to rack (n.1). Compare rack-bone "vertebrae" (1610s).