Ur mom is a simple comeback used in practically any scenario after a insult or just something that works with the joke..
Friend: Hey what are you doing after school
You: ur mom
You: hey man last night was so fun
Friend: what was last night
You: you mean you didn't hear me and ur mom in the bedroom??
Friend: hey whats up
You: ur mom
When someone says "Ur mom" they mean they hate you with all thier guts and would shank you with any chance they have
"You stole my food! Your such a Ur mom"
"Ur mom, god I hate you so much!!"
Person: I met the sexiest woman ever
Person2: who was it?
Person: ur mom
Ur Mom is a comeback for literally anything
someone: You look like a person that eat cereal with a fork
You: Then I guess I look like UR MOM
Someone: *dies*
Ur mom is a phrase used to be funny or comical, people often use it when being sarcastic.
guy 1: wow bro! Ur mom is a milf !
guy 2: no, ur mom is.