A ninja is someone who is very good at something. Usually, ninja would be prefixed by the skill or subject involved e.g.
- A wiki ninja is someone very good at editing the wiki.
- A scaley ninja is a very good Signaller.
Sometimes, ninja can be used as the prefix itself so you would have a Ninja Signaller.
There was a time in the 1980s when infantry privates who didn't get out enough, and spent too long watching cheap SSVC videos, believed 'Ninjitsu' to be a legitimate sport or past-time. Many of said infantry privates suffered severe fillings-in at the hands of their less gullible colleagues and abandoned the way of the dark and the fist...
The word ninja is itself Japanese, and means warrior, assassin or spy in Japanese culture, Skilled in the arts of stealth and killing folk, the martial art being called ninjitsu. Appearing in fourteenth century feudal Japan, and active from the Kamakura to the Edo period, Their roles may have included sabotage, espionage, scouting, assassination missions, perhaps in the service of feudal rulers, Daimyo or Shogun.
This is assuming they even existed, Ninja have the same sort of historical footprint as Robin Hood (other than some medieval chav with a bow taking potshots at anyone collecting taxes and picking up a 'rep').
Many feeble minded martial arts wannabes pretend to be ninjas. It appeals to the same people who think Katanas can cut through machinegun barrels - just look on YouTube for their postings!
Ninja would be the medieval japanese version of them!
The opposite of a ninja is a biff.