early 15c., surloine, from Middle 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]