The Collaborative International Dictionary of English v.0.48:

Sirloin \Sir"loin`\, n. [A corruption of surloin. Not so called
   because this cut of beef was once jocosely knighted (dubbed
   Sir Loin) by an English king, as according to a popular
   story.]
   A loin of beef, or a part of a loin. [Written also
   surloin.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Surloin \Sur"loin`\ (s[^u]r"loin`), n. [F. surlonge; sur upon +
   longe loin. See Sur-, and Loin.]
   A loin of beef, or the upper part of the loin. See Sirloin,
   the more usual, but not etymologically preferable,
   orthography. [Also spelled sirloin.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

sirloin
    n 1: the portion of the loin (especially of beef) just in front
         of the rump

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "sirloin":
   Chateaubriand, blade roast, breast, brisket, chuck, chuck roast,
   clod, cold cuts, filet mignon, flank, knuckle, loin, plate,
   plate piece, pot roast, rack, rib roast, ribs, roast, rolled roast,
   round, rump, rump roast, saddle, shank, short ribs, shoulder,
   shoulder clod, tenderloin