It is just to me a silly thing to say or just like Your mom is asleep.
your mother, or either a “comeback” that brain damaged kids use, sometimes they just reply with ur mom to everything cause of their brain damage.
carlos : hey julia do you know what’s for math homework?
nikita: ur mom
carlos: what?
nikita: ur mom
Something insulting people say when they don’t know what to say.
Hey Timmy where were you last night?
“Ur mom!”