(1.) The act of IMing someone as they're busy typing. The resultant pop-up window steals their cursor and often the victim continues typing into the return window unknowingly.
(2.) The justification for falling down for no apparent reason.
SniperIM: "Hey what time are you going to dinner?"
Victim101: “ntact for an appointment?”
Victim101: “sorry, you sniped me”
(Victim trips on the stairs)
Smart ass friend: “Sniper, everyone get down!”