noun, plural nin·ja, nin·jas. ( often initial capital letter ) a member of a feudal Japanese society of mercenary agents, highly trained in martial arts and stealth (ninjutsu) who were hired for covert purposes ranging from espionage to sabotage and assassination.
Origin:
< Japanese, equivalent to nin- endure + -ja, combining form of -sha person (< Middle Chinese, equivalent to Chinese rěn + zhě)