The story goes that King Henry VIII (he of six wives fame) appreciated his tucker as much as he appreciated a shapely ankle, and one night was so enamoured of his dinner that he knighted the cut of meat, calling it Sir Loin.
Great story, eh?
Sadly, like most good stories, it's not true.
Actually the name sirloin is a corruption of two French words used to describe the cut: sur (above) longe (loin).
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Definition: This cut of beef lies between the very tender short loin and the much tougher round. As would be expected, the meat cuts from the portion near the short loin are more tender than those closer to the round. Sirloin is usually cut into steaks or roasts. Bone removed, the cuts are referred to by the names of the three main muscles. Top sirloin is a continuation of the tender top loin muscle of the short loin. The tenderloin is part of the tenderest muscle (which also continues from the short loin) and the bottom sirloin, which is part of the same (less tender) sirloin tip muscle found in the round. The best-known bone-in sirloin steaks (in order of tenderness) are pinbone, flat bone, round bone and wedge bone. See also beef.
Source: the food network