Ur Mom

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