A quarter that you put between your crotch before you give it to someone.
Joe: Hey Mike! Can I borrow a quarter?
Mike: Sure, just give me a moment... (pretends to get quarter out of his pocket) Here you go!
Joe: Thanks! (walks away)
(You vaguely hear Joe say the quarter smells funny)
You: Do you think he'll find out that he got a Crotch Quarter?
Mike: Hell no! (laughs)