Etymology
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loin (n.)
early 14c., "side of the body of an animal used for food;" late 14c., "side of the lower torso of a human body," from Old French loigne "hip, haunch, lumbar region," from Vulgar Latin *lumbea, from *lumbea caro "meat of the loin," from fem. of *lumbeus, from Latin lumbus "loin," from PIE root *lendh- (1) "loin" (see lumbo-).

The native word was Old English lendenu "loins," from Proto-Germanic *landwin- (source also of German Lende "loin," Lenden "loins;" Old High German lenti, Old Saxon lendin, Middle Dutch lendine, Dutch lende, Old Norse lend). The Latin word perhaps was borrowed from Germanic. In Biblical translations, often used for "that part of the body that should be covered and about which the clothes are bound" (1520s), hence, in symbolic or figurative use, with reference to being the seat of sexual faculty and a symbol of strength. Related: Loins.
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loin-cloth (n.)
also loincloth, "cloth worn about the loins" (properly the hips), 1851, from loin (n.) + cloth (n.).
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tenderloin (n.)
1828, "tender part of a loin of pork or beef," from tender (adj.) + loin. The slang meaning "police district noted for vice" appeared first 1887 in New York, on the notion of the neighborhood of the chief theaters, restaurants, etc., being the "juciest cut" for graft and blackmail.
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sirloin (n.)

early 15c., surloine, from French surlonge, literally "upper part of the loin," from sur "over, above" (see sur-) + longe "loin," from Old French loigne (see loin).

English spelling with sir- dates from 1620s, by folk-etymology supposed to be because the cut of beef was "knighted" by an English king for its superiority, a tale variously told of Henry VIII, James I, and Charles II. The story dates to 1655.

The word surloin or sirloin is often said to be derived from the fact that the loin was knighted as Sir Loin by Charles II, or (according to [early 19c. English dictionary writer Charles] Richardson) by James I. Chronology makes short work of this statement; the word being in use long before James I was born. It is one of those unscrupulous inventions with which English 'etymology' abounds, and which many people admire because they are 'so clever.' The number of those who literally prefer a story about a word to a more prosaic account of it, is only too large. [Walter W. Skeat, "An Etymological Dictionary of the English Language," 1882]
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lumbo- 
word-forming element used since 19c. and meaning "loin, loins," from Latin lumbus "hip, loin" (usually plural), from Proto-Italic *londwo- "loins," from PIE *lendh- (1) "loin" (source also of Sanskrit randhra- "loin (of animals);" Old Church Slavonic ledvije (plural) "loins, kidneys, insides; soul," Russian ljadveja (archaic) "thigh;" Old English lendenu "loins," Old Norse lend, German Lende "loin," Lenden "loins").
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numbles (n.)

"edible viscera of animals, entrails of a deer," c. 1300, noumbles, from Old French nombles "loin of veal, fillet of beef, haunch of venison," from Latin lumulus, diminutive of lumbus "loin" (see lumbo-).

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lumbago (n.)
1690s, from Late Latin lumbago "weakness of loins and lower back," from Latin lumbus "hip, loin" (usually plural), from Proto-Italic *londwo- "loins," from PIE *lendh- (1) "loin" (see lumbo-).
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chop (n.)

mid-14c., "act of chopping, cutting with a quick blow," from chop (v.1). Meaning "piece cut off" is mid-15c.; specifically "slice of mutton, lamb, or pork" (usually cut from the loin and containing the rib) is from 1630s, probably from being "chopped" from the loin. Sense of "a blow, strike" is from 1550s. Specific cricket/baseball sense of "a downward stroke with the bat" is by 1888.

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lambada (n.)
type of sensual Brazilian dance, 1988, from Portuguese, said in some sources to mean literally "a beating, a lashing." But others [Watkins] connect it ultimately to Latin lumbus "loin" (see lumbo-).
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lumbar (adj.)
"pertaining to or situated near the loins," 1650s, from Modern Latin lumbaris, from Latin lumbus "loin" (see lumbo-).
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