In what context? The word 'lemon' has a host of meanings apart from the obvious one! Lesbian, penis, marijuana and pornographic story to name but a few.
It's just occurred to me, an hour and a half later, that it is probably just a typical bit of Cockney Rhyming Slang...lemon flavour/favour. Still, it could as easily have finished up as "Do me a mint" or "Do me an orange". However, most such rhymes could have been different. Why 'Apples and pears' for stairs when 'Rhinos and bears' would have worked just as well?