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Future brother in law. This designation can also extend to the siblings of the engaged couple. (See example)
Joel: when my brother proposes to your sister, we will be FBILs!

Aaron: best day ever!
by anjfbil February 07, 2013
an acronym for future brother in law
my fbil better watch himself before my bridezilla sister gets all crazy on his ass.
by TheRealCuddFoo February 04, 2011
Future Brother In Law
My FBIL is coming down this weekend
future brother in law FBIL
by TheRealCuddFoo February 04, 2011