Ya mum is a term commonly used by human beings for several humorous reasons.... The first of which can be dated back to the year of 1874 and is exactly this...
REASON 1: You are a
retard and have nothing more intelligent to say at this moment.
REASON 2: You're a funny kunt(or so you believe) and think you're
smart.
REASON 3:
YOUR MUM
....
...
..
.
See what I did there? I was(am) actually too drunk to Write anything decent as reason 3 because I am beyond thinking at this current time.