The name given to a man who carried out a series of attacks in Sheffield, England in the summer of 1988. These attacks involved him going door to door in residential areas throughout the city with a male chicken hidden discretely under his coat. Upon finding an unlocked door, he would forcefully thrust the "Mad Cock" on his victims on the other side of the door. Such attacks were marked with a card thrown into the fray with the words inscribed: "You've been Cocked by the Mad Cocker".
After my aunt was cocked by the Mad Cocker, she had to spend 6 weeks in nuthouse receiving daily electroshock therapy. She's still never fully recovered.