Norman's Cock was a battle during the invasion of Britain in 1066 by William the Conqueror. It was a particularly bloody battle, resulting in thousands of casualties on both the English and French sides. It is unknown where the name comes from, but it is believed to have been a misnomer from "Norman's Coq", ie, Chicken, the cowardly streak wrongly attributed to the French soldiers by the English.
Used slightly out of context these days, usually to gauge how bad something is.
When someone calls Marge "Duck", that's almost as bad as Norman's Cock.