What it means
A classic British brush-off for when someone's chatting absolute rubbish, making a daft claim, or asking for something with a straight face that deserves nothing but a hard eye-roll. Nobody wants an actual favour. It means come off it, give over, or don't take the piss.
Usage examples
"Do me a favour, there's no way that old banger does a hundred miles an hour."
"You reckon you ran it in under five minutes? Do me a favour."
"Do me a favour, mate, you did not pull a sickie and end up at the pub by lunch."
"He says he's never seen the message. Do me a favour, he was online five minutes ago."
"Do me a favour, you did not accidentally text your ex at two in the morning."
Where it comes from
Plain words turned inside out by tone. The literal request to do me a favour got so loaded with sarcasm that the dismissive reading took over, until barking it at someone became pure disbelief rather than any actual ask.
Other ways to say it
Editors of this term
Earthy Spark Tamer Official watcher of this expression
Tell me, what have you got for me?