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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.