The defenition of 'ur mum', coming from the ancient greek mythology as 'ureono mummus', is a term often used by people who try to convince the person they are talking to, as a form or manner to tell them they are 'gay'.
'Hey how are you doing?', friend 1 said.
'ur mum', friend 2 replied.
An idiotic saying used very often by little 8 year olds that don’t know anything else in their tiny vocabulary box but that. If you see someone that is older than that age call911 immediately.