1. A tasty fruit that makes you make a funny face.

2. A fan fiction that is based on an anime/manga or video game that is sexualy oriented.

3. Something that was useless or crappy.

4. A lame person or thing.

5. A bitter fruit.

6. A car that you buy that you think is good, but really it sucks like hell and will break right after you buy it.

7. an object of dubious value; a piece of crap.

(many more...)

http://www.urbandictionary.com/define.php

a motor vehicle with many mechanical problems.

http://onlineslangdictionary.com/definition+of/a+lemon

The term 'lemon' to describe a highly flawed item predates its use in describing cars.

http://en.wikipedia.org/wiki/Lemon_(automobile )

In American slang, a "lemon" is a car that causes more trouble that it is worth, as in "I bought a car from that dealership, but it turned out to be a lemon."

In Australian slang, a "lemon" is a lesbian (originally just a sour woman), as in "This town seems full of lemons."

In Cockney rhyming slang, a "lemon" is a favor -" lemon flavour" - as in "Gi' us a lemon, mate."

In fan fiction, a 'lemon' is a scene or story that includes explicitly described sex.