much like Milf, Gilf, Dilf(dad),Dilf (dog),Dilf(donkey)Cilf(child),Bilf,Cilf(cat)or any other farm animal.
Person 2: Dude, thats YOUR sister. That's nasty.
Person 1: It's called SILF for a reason dude. DUUU
A Sandwich made so perfectly one would be able to fornicate with previously mentioned sandwich.
Guy 1: "dude I just had the best S.I.L.F."
Guy 2: "What?"
Guy 1: "SANDWICH I'D LIKE TO FUCK!"