N. Term applied to hot (usually older) brothers of best friends (bfs) or aquaintences. Used when around said targets sibling or other friends who know of siblingship.
Akin and sometimes used in place of the rarer Filf (Family I'd like to fuck; i/e:cousin, step siblings ect., aunts, uncles, cousins inlaw - of friends)
Derivitive of Milf
Note: does not denote an incestual connotation.
Jess(studying, overtaken by jasons hotness): Your brothers a bilf.
Jess: Is he seeing anyone?
Lynn: What!?....your fucking crazy.
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.