From WoWWiki

A loot ninja or ninja looter, or simply ninja (on some servers, particularly in the EU, it is also called Pirate looting, as Pirates were renown for stealing, which seems like a more logical name for the term), 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 (that is, with ninja-like speed). The term seems to have a few different meaning depending on whom you ask, including:

• a player who, when in a group, rolls "Need" on everything regardless of if he or she can use it, also known as (despite its name) a greeder,
• a player who takes everything not nailed down in an instance without asking: chests, ores, skins, herbs, etc.,
• a player who rolls on an BoP item that everyone else has passed on or a player who loots a corpse without permission after everyone has passed on a BoP item. Generally groups decide how to best make use of the item that no one can use, sometimes dps classes will back out of +healing/+spell damage (as opposed to just +spell damaqge) even if it is an upgrade and will ask for the item if no one else needs it. Additionally, if no one needs it and no one reconsidered, it often goes to a enchanter, who disenchants it, and then the group rolls for the materials acquired from the disenchanted item. Sometimes players loot items for more sentimental reasons (like "town clothes" in RP servers) and will normally pass on it unless no one else needs it. If there are no enchanters in the group and no one changes their mind, generally the group will roll for it's NPC value.
• a player who loots a chest or harvests a resource while another player is fighting the mobs which were defending it, possibly using stealth or a dash ability to aid in this, and
• a party or raid leader who changes the looting system to "Master Loot" near the end of a boss kill and steals the loot off of a corpse without letting other players roll for the items. Especially one who sets up a group just for the first or second boss, and then hearthstones without speaking or claiming he has to sleep or work, meanwhile setting up another group to loot it again, perhaps prominent of Gold Farmers.
• A player's reputation can go down if they continuously win upgrades over other players. It is not uncommon for people to whine if a player has won 3 out of 3 items for their class, sometimes being labeled as a roll hacker. The worst case is generally 2 greens drop from trash, both are upgrades to the same slot, and the player equips (and therefore binds them to him) them. Later in the raid or instance a BoP Blue drops from a boss, and the same player wins that item (thus binding it to him). In this situation the party may be displeased as not only could the greens have been sold on the Auction House but it is very likely they could have upgraded two other players with the same role.

Gold farmers also try to join instance groups in order to ninja/pirate loot items and sell them.

Loot ninjas are, understandably, not well liked by other players. Often, a ninja will get a bad reputation on a realm that eventually prevents them from being invited to participate in any high level instances. Many guilds also have rules stating that ninjas/pirates are kicked from the guild on first offense.

World of Warcraft Game Design

Several features in World of Warcraft were designed to minimize the opportunities for players to ninja/pirate loot, addressing problems which had occurred in earlier games.

• A mob is tagged by the first player character or group to damage it, and it can not be looted by anyone else.
• Rogues can not remain stealthed while looting or harvesting resources.
• The loot options in the game allow a person to roll on items that drop. While a ninja/pirate may roll on items he or she shouldn't, there is a chance the more deserving player will win.