The idea that you are entitled to having relations with a specific girl simply because your brother, cousin, best friend, co-worker, fraternity brother (or sorority sister, if that so happens to be your thing), or roommate hit it.
"His little brother in the fraternity called me 'cutey'"
"I don't know! It was weird because we've never actually spoken."
"Maybe he thought he could get pussy by proxy."
"Pussy by proxy?"
"Yeah, his big brother probably told him that he hit it, so now he wants to as well and thinks it will be easy because he's his little."