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1.
A humerous and witty retort aimed at people who offend you using the popular slang "your mum" which can have deep and hurtful meanings to the receiver often resulting in them getting annoyed and wanting to fight.

This line asks the receiver to visualise their own mother ona slice of toast, often with side fillings such as cheese , the funnier more extreme the better.
"Your mum!"
"Yeh? well your fuckin mum on toast with cheese and lettuce smothered in tomato sauce!"

"hey dude you fuckin suck"
"Your mum on toast bitch"
by GF June 28, 2006
 
2.
A humourous elaboration on the famous "your mum" joke - see "your mum"
- "what time is it?"
- "your mum!"
- "your mum on toast!"
by Phinycs November 01, 2005