Definition
Expresses disbelief.
Origins
"Do me a favour" (rhyming with "lemon flavour") in Cockney rhyming slang.
Also Said As
How People Actually Use It
Means literally 'do me a favour'. The lemon part is Cockney rhyming slang, and is short for 'lemon flavour', so in other words it's really "do me a lemon flavour" The usage is mostly to reject an offer or behaviour of some kind, due to its being undesirable in some way (as opposed to actually asking for a specific favour).
""Do you want to buy my iPod for 100 quid?" "Do me a lemon. I could get one on ebay for 50.""
Source: Urban Dictionary