It refers to a brother of a friend that is very attractive, which you don't mind having sexual intercourse with. Gay black people may use this, when referring to hot guys which they see on the street, because to a black man everybody is their brother.
Steve: So he stole all your shit?
Steve: What a bilf
You bilf nugget
you bilfing moron.
I'm gonna fill you with so much bilf you're going to get swimmers ear.