Common way of ending
arguments or
discussions in
England. Example:
- I think you're wrong.
- Well, your mum!
- No, your mum!
- Your mum on toast!
- Your mum and a TV!
... and so on.
The discussion can now go on at a safe level. It has deteriorated into something similar to "my dad is stronger than your dad", only less intellectual.
Presumably, this rhetorical trick comes from insulting the other person's mum, as in "Your mum's a slag", or from more general insults like "no one likes you, not even your mum", but preferably the insults are implicit or nonsensical (see example above).