Frag is a commonly used term in first-person shooter game. When you kill an opponent in the game you call it a "frag" rather than kill. The term frag is commonly associated with multiplayer deathmatch games.
The word frag was first used in the Vietnam war, but there is some discrepancies as to the meaning of word from this time. Some references point to it being used to describe a soldier who was brutally killed by a grenade, while other resources state "to frag" is a term used in wartimes to describe the use of a fragmentation grenade "to cool the ardor of any officer or NCO too eager to make contact with the enemy".
See also deathmatch.