It varys from person to person but in the end has a vary similar meaning.
In short it is a douchebag, also can be used when someone is an asshole, dickwad, cunt stain, rotten taint, sleeze bag.
Can also be a way to classify people who are epicly parinoid about pretty much anything.
If this person exhibits both of the above then they are a Butkowsky.
You fucking butkowsky!
Dude your such a butkowsky.
Dude stop acting like a butkowsky! I never took your pencil.