is commonly used as a comeback and jokes. It means exactly what it sounds like, but please do not use this one when talking to a sibling it is not the same and ruins it.
person 1: what were you doing today?
person 2: ur mom
sibling 1: what were you doing today?
sibling 2: ur mom
sibling 1:... we have the same mom...
sibling 2: yeah sorry
Quite the powerful comeback. So much so, that in an argument, it cannot be countered. Your opponent will drop dead after they hear it, so use it wisely.
Gary: YOU'RE SUCH A DOLT
You: UR MOM
Gary: *dies*