Found this on Webster's

Main Entry: Manning

Pronunciation: \ˈskrü\

Function: noun

Etymology: Middle English Dufus, from Medieval Latin Dumbass, from Latin, dipshit

Date: 15th century

1 a : a simple dufus that incurs the play that screws a team from coming back and winning a game.

b : a nail-shaped or rod-shaped piece with a spiral groove and a slotted or recessed head designed to be inserted into the ass of all your teamates by rotating (as with a screwdriver) ) 2 : a worn-out useless sack of shit 3 chiefly British : a small packet (as of tobacco) 4 : a prison guard who gets it

Closely related but not to be confused with a Favre, a Favre is when you actually lose the game with a turnover. A Manning is sealing the deal of screwing your team totally from winning.

Ex: He pulled a Manning last night by pissing on the bouncers leg, thus ensuring to have his ass kicked.