Etymology
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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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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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lumber (n.)

"timber sawn into rough planks for use," 1660s, American English (Massachusetts), earlier "disused bit of furniture; heavy, useless objects" (1550s), of uncertain origin. It is said to be probably from lumber (v.1) on the notion of "awkward to move," and perhaps to have been influenced by or associated with Lombard (q.v.), the Italian immigrant class famous as pawnbrokers (and money-lenders) in old England. Lumbar and Lumbard were old alternative forms of Lombard in English.

The evolution of sense then would be because a lumber-house ("pawn shop; place where thieves stash stolen property") naturally accumulates odds and ends of furniture. The 19th century guess was that it comes directly from lumber-house or lumber-room in the pawn shop sense, but these are not attested before lumber (n.). Lumber camp is from 1841; lumber-mill is from 1830; lumber-yard is from 1777.

Live Lumber, soldiers or passengers on board a ship are so called by the sailors. [Grose, "Dictionary of the Vulgar Tongue," 1785]
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belly-dance (n.)

also bellydance, 1883, in a British account of travels in Persia, from belly (n.) + dance (n.). In early use sometimes referred to by the French danse du ventre, which is attested by 1872 in French accounts from the Middle East. It appears as a French term in English by 1883, and its use got a boost from the performances of it at the Paris Exposition of 1889.

We agreed, and made our way to the mimic street called Grand Cairo, where we witnessed the lady contortionist who performs a series of movements, designated with charming frankness on the affiches as "La Danse du Ventre." It might with equal candor be called the Lumbar Wriggle [or] the Pectoral Squirm, for this curious Arab almeh possesses the power of moving any one of her principal sets of muscles quite independently of all the others, and can make any prominent part of her person waggle or surge, while its neighboring lines or curves preserve a statuesque rigidity. [Table Talk, September 1889]
The number of women [in the audience] was ludicrously disproportionate, and the number of American women was noticeable. Some of them seemed slightly pensive, but all were interested. Their large eyes grew larger still. They almost forgot decorum in crowding for a better view, in leaning over the backs of chairs in concentrated absorbed attention. [Scribner's Magazine, January 1890]

The English noun is perhaps a direct translation of the French. As a verb from 1963. Related: Belly-dancer (1922); belly-dancing (n.), 1921.

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