Ninja Cum: (1) The act of cumming inside of somebody without them ever knowing. (2) Also, the act of cumming on an unsuspecting victim without them knowing.
(1) Girl: Well that was fun, sorry you didn't finish Guy: I liked it ... one week later. Guy to Friend: I totally ninja came (past tense of ninja cum)
(2) Person 1: (time to ninja cum) *jerk, jerk, jerk* Person 2: *yawn* Person 1: *SPPPLOOOOOOSSSSHHHHH* Person 2: *yawn* Person 1: *silent zip* well, see you later. Person 2: bye