look up any word, like cleveland steamer:
 
1.
An epiphany of thought. When someone suddenly has a brilliant idea.
omg, i just birthed a brain baby.
by Volcanis July 19, 2011
 
2.
The result of when a man jizzes in a girl's ear
My girlfriend didn't want me to cum in her mouth, so I came in her ear. In 9 months, she'll be having a brain baby.
by wotcher now October 20, 2010
 
3.
Anything with an overly large head, or anything that looks top-heavy in any way.
Man, did you see that psychobilly, brain baby thing over there? it's head was all greasy and stymie like rubber johnny's.
by Angus_for_president November 26, 2007