Many first-person shooter games have teleporters. Telefragging was afaik invented in DOOM, where if you teleport into space already occupied by a monster or player, you take over that space, fragging the monster or player.
In the final map of Quake, the way to kill the boss is to enter a teleporter when the other end of that teleporter enters the boss, thereby telefragging it.
v. A concept derived from video games that have some kind of teleportation element. If the space that something would teleport to (or respawn to) happens to have something else already there, one of 5 possibilities could occur: the object in that space would be destroyed, the object doing the teleporting would be destroyed, both objects are destroyed, the objects get merged in some fucked up deformed way, or nothing at all. The concept is also be used in science fiction, for example: in Rick and Morty if a portal is closed on an object halfway through it, or in Battlestar Galactica when a miscalculated FTL jump lands a ship inside a planet or star.
I got bored of all the weapons so now I just use the Translocator to telefrag n00bs.
In Halo, to attempt to continue a teleport while another player is standing on the tele pad, despite warnings of "The Teleporter is Blocked"
The player on the teleport is then killed, dubbed a 'Telefrag.'
Taken from Unreal Tournament.