When your sister wants you to fuck her. In most cases, brothers ask their sisters. But that's probably the only thing that sibs can agree on...
sex love sisters brothers ass yummy tits penis dick vagina seducer
Sister: Bro, come here for a sec...

Brother: What?

Sister: (strips off clothing)

And they lived happily ever after..
But she's a guy stealing SILF!
Substitute I'd Like (to) Fuck
Johhny: "Did you see that math substitute today?"

Mark: "Yea, dude, she's a total SILF!"
by jjobin@99 February 14, 2011
Baily and/or Ridgly (Sisters of Smapdey). Also see the The B&R
Boy, smap's sister's are deffenately silfs. Especialy those tits and edible training bra!
by Chester January 11, 2005
Sister I would like to fuck.
much like Milf, Gilf, Dilf(dad),Dilf (dog),Dilf(donkey)Cilf(child),Bilf,Cilf(cat)or any other farm animal.
Person 1:OH MY GOD, look at that chick.What a SILF.
Person 2: Dude, thats YOUR sister. That's nasty.
Person 1: It's called SILF for a reason dude. DUUU
by ill bang ur sister! December 8, 2004
dude, shack had some serious silf last night. He's still pissed off
by i banged your mom October 19, 2008