ninja

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A member of a class of medieval Japanese mercenary agents who were trained in the martial arts and hired for covert operations such as assassination and sabotage.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun martial arts, historical A person trained primarily in stealth, espionage, assassination and the Japanese martial art of ninjutsu.
• noun A Mongolian amateurish private miner (mainly for gold); after the shape of the plastic bowls used to wash metal ore with mercury, roughly resembling one of the Teenage Mutant Ninja Turtles.
• noun slang Juggalo version of the epithet "nigga".
• verb To act in the manner of a ninja, especially in the areas of speed and power.
• verb gaming, slang To claim an item in a game by abusing game mechanics, often despite having no real need for the item or ability to use it.
• verb Internet, slang To post a response on a message board immediately before someone else unknowingly posts a response saying the same thing.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a class of 14th century Japanese who were trained in martial arts and were hired for espionage and assassinations
• noun a member of the ninja who were trained in martial arts and hired for espionage or sabotage or assassinations; a person skilled in ninjutsu