The word ninja comes from two Kanji characters.
They can be read in two different ways.
Native Japanese or a Chinese - derived word.
Native Japanese the characters read shinobi and mono. When these two words are out together you need to add a linking word no. Thus it translate to shinobi no mono.
Chinese derived word, the character translate nin and sha. Pronounced, you guessed it, Ninja.
Now the words shinobi and nin mean "stealth" or "quiet action". But they also mean "to endure"
Sha and mono both mean "person"
So the Kanji characters if you read them in either native Japanese or the Chinese derive word meaning "a person skilled in stealth" or "one who endures."
How cool is that?
Be stealthy and endure.