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1.
A girl that has a nice body and a beautiful face but she's dumb as shit.
Lauren is so fucking hot but she is a huge butter brain.
by Lakitaarii January 17, 2011
 
2.
A female who is super hot with a nice body and face, but is brain dead.

butterbrain = "but her brain"
Guy 1: Damn that girl is smokin hot!
Guy 2: Yeah but she's a butterbrain
Guy 1: Oh screw her
by Makolyte June 26, 2009
 
3.
1.) (n) Someone who uses drugs regularly and/or habitually, esp. hard drugs (crack cocaine, heroin, methamphetamines, etc.)

2.) (n) A junkie or drug addict.

Called a "butterbrain" as the user's brain has been melted down or fried through constant drug abuse. This shows itself through a lack of rational behavior, thought process, and severely limited intellect.
1.) "Man, that dude has smoked so many rocks, he's just a butterbrain now".

2.) "That dealer down the block is always sold out from those butterbrain's buying up all his shit."
by Paul W. G. June 05, 2007