A person, usually male aged 13-35, who practices inferior self-taught fighting, killing, or stealth techniques in the basement of his/her parents' home or in a basement apartment. Typical hobbies include collection of decorative 'ninja' weapons for the purposes of practice and display. Typical behaviours include exhibition of martial arts proficiency, provision of stealth tips, and demonstration of human pressure points.
"Anybody who carries nunchucks to a 7-11 is a basement ninja."
A person(usually a teenager) who rarely sees the light of day and spends his time playing videogames ranging from: Modern Warfare 2 to World of Warcraft.
Me: Dude, I was enjoying a casual game of Modern Warfare 2 when this 10th Prestige dropped into my game and fucked everyone up.
Friend: What a fuckin Basement Ninja.