A person with the ability to insert thoughts or ideas into another's head that does not cause immediate, but rather long term lingering mentalanguish, usually having comedic value.
Walk up to someone. Look at them thoughtfully for a moment. Sing, "I've got a golden ticket." Walk away. BAM, you're a brain ninja.
Jess: We were having a fight bout (something stupid that was Jess' fault) but i totally turned it around and made him feel bad bout it.
Ashlee: wow, you're such a brain ninja jess!
Title given to girls by comedian Dane Cook. Girls have the power to fuck with a guy's mind by saying something so simple that it eventually drives them crazy.
Girls are brain ninjas! They know how to get in there with their katanas and cut ya! And then disappear into the night!
when your in a horrible "relationshit" and you get in a fight one day and the girlfriend happens to say a comment that will seem at first like the worst come back ever but soon will make you so friggin' angry that you go make a ass out of yourself in front of her
yeah steve i completely was fuckingbrain ninja'd, i walked in and just sounded like a complete dumm shit