Meaning

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

noun a class of 14th century Japanese who were trained in martial arts and were hired for espionage and assassinations

originA romanized borrowing of Japanese 忍者 (ninja), popularized within Japanese by manga after World War II and in English by Eric Van Lustbader's 1980 novel The Ninja and the 1981 movie Enter the Ninja, of uncertain derivation but with an underlying sense of secret or hidden person. The “Mongolian miner” sense arose from the supposed resemblance of the bowls used to wash ore with mercury to the shells…

Etymology from Wiktionary, CC BY-SA 3.0