c. 1300, "grating or open frame with bars or pegs upon which things are hung or placed," especially for kitchen use, 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- (source also of Old Saxon rekkian, Old Frisian reza, Old Norse rekja, Old High German recchen, German recken, Gothic uf-rakjan "to stretch out"), from PIE root *reg- "to move in a straight line."
Or it might have developed from the Old English verb. The sense of "frame on which clothes or skins are stretched to dry" is by early 14c. The sense of "framework above a manger for holding hay or other fodder for livestock" is from mid-14c. As the name of a type of instrument of torture by early 15c., perhaps from German rackbank, originally an implement for stretching leather, etc. Sense of "punishment by the rack" is by 1580s.
Mechanical meaning "metal bar with teeth on one edge" 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, between a trot and a gallop or canter, 1580s, from rack (v.) "move with a fast, lively gait" (1520s, implied in racking), which is 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, rak, "movement, rapid movement," also "rush of wind, collision, crash," originally a northern word, possibly from Old English racu "cloud, storm" (or an unrecorded Scandinavian cognate of it), reinforced by Old Norse rek, *rak "wreckage, jetsam," or Old English wræc "something driven," both of which would be from Proto-Germanic *wrakaz, from PIE root *wreg- "to push, shove, drive" (see urge (v.)).
From late 14c. as "rain cloud." Often confused with wrack (n.) "destruction," especially in the phrase rack and ruin (1590s), which perhaps is encouraged in that case by the visual alliteration. Rack is "fragments of raggy clouds;" wrack is, in its secondary sense, "seaweed cast up on shore." Both probably come, ultimately, from the same PIE root, as does wreak.
early 15c., rakken, "to stretch, stretch out (cloth) for drying," also, of persons, "to torture by violently stretching on the rack," from rack (n.1) or from Middle Dutch or Middle Low German recken. Of other pains from 1580s.
Figurative sense of "subject to strenuous effort" (of the brain, memory, etc.) is by 1580s; that of "to torment, afflict with great pain or distress" is from c. 1600. Meaning "fit with racks" is from 1580s.
Sense of "to place (pool balls, etc.) in a rack" as before starting a game is by 1909 (the noun in this sense is by 1907). 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 attested by 1943 (in Billboard magazine), probably from pool halls, perhaps from a method of keeping score. To rack up formerly was "fill a stable rack with hay or straw for horses kept overnight" (1743).
"cut of animal meat and bones," usually involving the neck and forepart of the spine, 1560s, of unknown origin; perhaps from some resemblance to rack (n.1). Compare rack-bone "vertebrae" (1610s).
"to draw off (wine, etc.) from the lees; draw off, as pure liquor, from sediment," late 15c., rakken, from Old French raquer, rëechier, rëequier which is of uncertain origin. Related: Racked, racking.
"extortionate rent, rent raised to the highest possible limit, rent greater than any tenant can be expected to pay," especially of land-rents in Ireland, c. 1600, from rent (n.) + rack (v.1) in the otherwise obsolete sense of "extort or obtain by rapacity, raise (rent, etc.) above a fair level" (1550s).