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...
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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!
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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
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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
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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
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dude, shack had some serious silf last night. He's still pissed off
by i banged your mom October 19, 2008
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