"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).
Entries linking to rack-rent
[payment for use of property], mid-12c., in a legal sense, "compensation made periodically, with reference to time of possession and use of property," from Old French rente "payment due; profit, income" and Medieval Latin renta, both from Vulgar Latin *rendita, noun use of fem. past participle of *rendere "to render" (see render (v.)).
It came into English earlier in a more general, and now obsolete, sense of "income, revenue" (late Old English). The sense in political economy, "what is left from the produce of the soil after deducting what is necessary to support the producers, interest, seed-corn, etc.," is by 1815. Rent-free is attested from 1630s.
Rents (to think how much of evil there is in the two senses of that four-lettered word ! In the two methods of intonation of its synonym, Tear !) [Ruskin, "Fors Claveriga"]
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).
"loud, disorderly, confusing noise," 1560s, probably imitative. Klein and Century Dictionary compare Gaelic racaid "noise, disturbance," but OED says this "is no doubt from Eng."
Meaning "dishonest activity" (1785) is perhaps an extended sense, from the notion of "something going on" or "noise or disturbance made to distract a pick-pocket's victim." Or it might be from racquet, via the notion of "a game," or from or reinforced by rack-rent "extortionate rent." There also was a verb racket "carry on eager or energetic action" (1753), and the gangster sense might be via the notion of "exciting and unusual." Weakened sense of "way of life, one's line of business" is by 1891.