King Arthur was worried about leaving Queen Guinevere alone with all those horny knights of the Round Table. So he went to Merlin for some advice. After explaining his predicament to Merlin, the wizard looked thoughtful, and said that he'd see if he could come up with something, and asked him to come back in a week.
A week later King Arthur was back in Merlin's laboratory where the good wizard was showing him his latest invention. It was a chastity belt... except that it had a rather large hole in the most obvious place.
"This is no good, Merlin!" the king exclaimed, "Look at this opening. How is this supposed to protect m'lady, the Queen?"
"Ah, sire, just observe," said Merlin as he searched his cluttered work bench until he found what he was looking for. He then selected his most worn-out wand, one that he was going to discard anyway. He then inserted it in the gaping aperture of the chastity belt whereupon a small guillotine blade came down and cut it neatly in two.
"Merlin, you are a genius!" said the greatful monarch, "Now I can leave, knowing that my Queen is fully protected."
After putting Guinevere in the device, King Arthur then set out upon his Quest. Several years passed until he returned to Camelot. Immediately he assembled all his knights in the courtyard and had them drop their trousers for an informal 'short arm' inspection. Sure enough, each and every one of them was either amputated or damaged in some way. All of them except Sir Galahad.
"Sir Galahad", exclaimed King Arthur, "The one and only true knight! Only you among all the nobles have been true to me. What is it in my power to grant you? Name it and it is yours!"
But Sir Galahad was speechless.