Green’s Dictionary of Slang

secretive; also as n. a secretive person.

1997–2000		College Sl. Research Project (Cal. State Poly. Uni., Pomona) 🌐 Ninja (adj.) Secret; on the d.l.
2003		Dizzee Rascal in Vice Mag. at Hyperdub.com 🌐 Real shotters, proper drugs dealers who you can’t really see, go about ninja.
2012		C. Eble (ed.) UNC-CH Campus Sl. 2011 8: NINJA — undesirable person who sneaks to the top of a group: ‘Herman Cain is such a ninja.’.