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]