Turning up unexpected and unwanted at a party, house or gathering, etc
Having a well cool party till these chavs gatecrashed
To "accidentally" insert your penis into a female companion's anus during vaginal intercourse. Often performed during doggie style
Mike F: What happened to that sidepiece you were seeing?
Ty: We broke up. I totally gatecrashed her the other day and she wasn't having it. She didn't believe it was an accident.