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.
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]
"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-).
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.