The phrase "Your Mum" - where on earth did it originate?

The other day, I was on the bus, and overheard the following exchanges: "Yuk! What stinks?" "Your mum." "Your face." "Your mum's face." Then there was this one: "What's the time?" "Your mum." And how about this gem? "Hey, I... show more The other day, I was on the bus, and overheard the following exchanges:
"Yuk! What stinks?"
"Your mum."
"Your face."
"Your mum's face."

Then there was this one:
"What's the time?"
"Your mum."

And how about this gem?
"Hey, I got an iPhone last week."
"Your mum got an iPhone last week."

Weird stuff. It seems that kids use "your mum" as a response for all kinds of things.
But where does this oddly ubiquitous phrase come from?

And why don't they ever say "your dad?"

Can you give more examples of this phrase's usage?

Cheers,
Hafwen x
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