This term generally applies to MMORPG's, and is used to describe someone who will join a party for the sole purpose of taking every single item of any value whatsoever, regardless of who killed the target and is the rightful owner of the items. A loot ninja will most often seem like a normal player, except when time comes to play the number roll game for a rare, they will roll, and then exploit a bug or a glitch in which they take the loot, regardless of what they rolled. Often hated by other players, and even more so by others of their kind. Sometimes referred to as a Loot Whore, or a n00b. Most people consider loot ninja'ing and those who do it to be committing a high crime, and often bring in a game master about them.
Party member 1: Did OMGHAXX take the rare again?
Party member 2: Yeah, he's such a f***ing loot ninja dude
System: OMGHAXX has obtained -Insert rare item-
System: OMGHAXX has left the party
One who takes advantage of online players who cannot type as fast and/or do not have a voice headset to speak on, and kills them while they are in the process of typing out a message. Not to be confused with a TK'er, which means something totally different.
Victim: *Typing out message*
Attacker:*Takes adavantage of victim's inability to protect themself and kills them*
Victim: Dude, stop type killing, ya Lamer.