loin (n.) Look up loin at Dictionary.com
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.