Snopes.com fact checks and deals with the etymology of the word thus :
While it is certainly possible that one or more England monarchs has repeated this pun, the word “sirloin” first appeared in English as far back as the early sixteenth century and therefore (with the possible exception of Henry VIII) antedates the reigns of any of the kings commonly cited as having originated it by knighting a piece of beef (e.g., James I, Charles II).
The real death blow to the “sir loin” etymology, however, is the fact that the word sirloin was originally written as “surloyn” or “surloine” in English, reflecting its origins in the Middle French word surlonge (“sur la longe”), with sur meaning “over” and longe meaning “loin” — hence sirloin was a cut of beef taken from above the loin. Not until considerably later did the common English spelling of the word shift towards using an “i” in place of the “u,” thus giving license to generations of punsters.