1. a Sister I'd Love to Fuck
2. the name for a person's hot sister that you would take pleasure in having sex with.
Bob's sister Allison is a total S.I.L.F.
Sandwich I'd Like To Fuck
A Sandwich made so perfectly one would be able to fornicate with previously mentioned sandwich.
"Oh man, that was such a good sandwich I'd basically call that a S.I.L.F."
Guy 1: "dude I just had the best S.I.L.F."
Guy 2: "What?"
Guy 1: "SANDWICH I'D LIKE TO FUCK!"