specifically used as a sly response to people asking questions u don't know the answer to.
idiot #1: hey Joe, what's the state capital of Nebraska?
smarter idiot #2: ehem... ur mom.
It is a joke about your mom. It is used to insult peeoples moms. YOU MOM
She's your Mom, and her name is Ur Mom. She is definitely, probably, totally Ur Mom.
Billy:Hey how are you man
Johnny boi: Ur mom
Billy: I am internally dying
the most op comeback that can even destroy god
jim: hey what did you do last night?
pablo: ur mom
jim: *dies*
jims mom: *dies*
pablo *alives*