1) One, usually male, who engages in anal sex with another male, but who exhibits ninja-like speed, agility, or cunning during fornication.
2) A mostly derogotory term in the non-homosexual vernacular, something one straight man would say to degrade or embarass another. See also Butt Pirate or Ass Jockey.
1) We just kept moving from position to position, it was like he was some sort of bum ninja!
2) Oh man, what are you? Some bum ninja? Take a drink!