A person with the ability to insert thoughts or ideas into another's head that does not cause immediate, but rather long term lingering mental anguish, 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.
someone who has the ability to get into your head and fuck with your mind.
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!