When you aren't feeling it "down there", but your brain(and nervous system) is jstu so overloaded with feeling(usally about a person).
girl one: omg..look at the hot guy!
girl 2: yah..thats michael!
girl 3: wow...omgomgogm! brain ORGASIM!
When you're illiterate and you don't know how to spell Brain Orgasm
Timmy tried to define 'brain orgasim'... he didn't know he spelled it incorrectly. Ironic because it has the word Brain in it, inferring thought and logic.