What is a loot ninja?
A loot ninja, aka ninja looter or simply ninja, is a player who takes loot to which he or she is not entitled. The act is referred to as ninja looting or ninjaing. The term predates World of Warcraft and originated with the notion of looting as quickly as possible. The common synonym for this behavior is greeder, though that term has ironically fairer play connotations specific to WoW. Source: Loot Ninja - WoW Wiki