What is the etymology of the word ninja?
In Japanese, the Kanji alphabet uses mostly Chinese characters
that represent ideas instead of sounds. Therefore, each kanji
character can be read phonetically either in Japanese or
Chinese.
The word 'ninja' comes from the Chinese characters 'nin' and
'sha' (which basically mean 'stealth' and 'person').
Those same characters, in Japanese kanji, are read as 'shinobi
no mono'.
So the Japanese word for 'ninja' is 'shinobi'.