ur mum is what people say when they don't want to say an actual answer.
I had passionate sex with ur mum last night.
Friend 1: What day is it?
Friend 2: ur mum
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"
ur mum is often used by a British person. Also is often used as A JOKE or a INSULT
person 1: what time is it ?
person two: time to fuck ur mum
person 1: ur clapped
person 2: I was making clapping noises with ur mum last night shut up fam
The ultimate insult to destroy your enemies weaknesses: no u
Fred: ur gay
Bob: do you know who else is?
Fred: no
Bob: ur mum
Fred: (gets sniped)