Skills relating to espionage and assassination were highly useful to warring factions in feudal Japan. Because these activities were seen as dishonorable, Japanese warriors hired people who existed below Japan’s social classes to perform these tasks. These persons were literally called “non-humans” [...] At some point the skills of espionage became known collectively as ninjutsu. And the people who specialized in these tasks were called shinobi no mono. Somewhat later they were called ninja.

Today’s martial arts fun fact source