A term used in Halo 3
Team Multiplayer matches. When an opponent has no shields and is on the verge of death. All it takes is one more shot to finish him/her off.
1st Guy: Over at sniper tower, he's oneshot.
2nd Guy: Got em.
1st Guy: Niiiiice.
1st Guy: WTF dude, why didn't you finish him off!? He was oneshot!!
2nd Guy: I didn't see him you f*cking *sshole!