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.