Lemon (n) - A worthless thing, like a car. Dates to 1909 in American slang. Possibly from an older pool hall hustle, the "lemon game."
Lemon Game (hustle) - A hustler gets a mark into a pool game, playing for credit. The hustler loses a significant amount of money, say $100. When it comes time to pay up, the hustler refuses to pay unless the mark displays $100 to show he could have paid off if he had lost. When the mark gets the money he is robbed.
Lemon Tree (folk song written by Will Holt in the 1960's)
When I was just a lad of ten, my father said to me,
Come here and learn a lesson from the lovely lemon tree.
Don't put your faith in love, my boy, my father said to me,
I fear you'll find that love is like the lovely lemon tree.