1. La Porta
2. to strike something

1. Don't let the door hit ya, where the good lord split ya!
2. Good riddance to bad trash

This saying has two usages. One would be rude, but you might use it with a a person who is a pain in the ass. It has a sarcastic tone to it, so it wouldn't be used in the place of something emphatic like 'Get out' or 'Leave'. But more as a reference to the fact that someone is leaving and you are pleased with that. The full saying would be 'don't let the door hit you in the ass on the way out'. You would also use this with friends. Here is used to be silly. There would be a sense of familiarity with the person you would use this with, such as a girlfriend/boyfriend or best mate, someone who knows you're being a smart-ass.