This can be defined in many ways. One of those being a person that steals your kill. That is the most basic, but there is also a person that comes in after a battle and pumps about one round of SMG into you and you die. Another definition of mooch is a person that takes all of your answers and somehow ends up getting a better grade than you.
Mooch:n:The biggest noob that every one hates and needs to learn to play a game as a legit person or just get your ass off of the game.
"You goddamn mooch!!!! I shot you like ten thousand times!!"
"Uh oh here comes _______ hes gonna kill you with the ultimate noob combo"
"I swear if you mooch this kill I wil rape your ass so hard that you will never return to this game again!"