Ur mum is used to describe a hot person such as your mother, it is a complment and means alot of love.
Ur MUM is so hot
People use this as a joke as making fun of people's mothers by simply saying ur mum
"Yo I banged ur mum last night"
1- “holy shit, whats that big fat monster?”
2- “what?? where?”
1- “oh. its just ur mum”
basically every young kids say to people as a joke
"what the fuck are you looking at" "your mum"