nin·ja 
(nĭnjə)
(nĭnjə)n. pl. ninja or nin·jas
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.
[Japanese : nin, to endure (from Early Middle Chinese ᶮin'; also the source of Mandarin rěn) + sha, person (from Middle Chinese tʂia´, pronoun for head of a relative clause; also the source of Mandarin zhě).]