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."