Ninja

ninja, n. 1. in medieval Japan, persons of certain clans, trained in ancient Japanese martial arts and employed especially for espionage and assassinations; the correct plural is ninja; Japanese, from nin- "persevere" + -ja "person." 
2. in games, particularly those with fantasy, East Asian, or cyberpunk milieu, characters based upon the ninja of history, usu. martial-arts assassins. 
3. (excl.) used in response to any particularly deft or sneaky move."
Ex: "You drop out of the skylight, land on the shoulders of the guards, knock their heads together, and flip onto the table, avoiding both the pressure-sensitive plate and the laser scanners." "Ninja!" [GJ1]
4. in MMORPGs, someone that joins a group temporarily in order to simply take treasure as quickly as possible, then leaves unexpectedly. (also with the verb "to ninja" 
Ex: "Don't play with BobZilla, he ninjas everything")
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Addendum: Modern ninjitsu often claims to be rooted in Japanese history, tracing back to the clans of ninja. However, this claim is generally controversial, as many point out that the aincent ninja clans had died out.