Source : Webster's Revised Unabridged Dictionary (1913)

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}.]

Source : WordNet®

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