A crotch hopper is a person, usually a woman, who is very promiscuous. She loves to hop from one crotch to another. A crotch hopper is similar to a crotch cricket except when a crotch hopper rubs her legs together, you can tell how many crotches she's hopped.
Dave: Dude, Daisy is such a crotch hopper!
Mike: You ain't shittin. Wait, she's rubbing her legs together!
Dave: Let me calculate this...thats two, carry the one, plus five. Damn dude, she's slept with 1200 guys.