A polite insult to a dumb/naive person, kind of like saying sweetie but in a more strict manner.
Fruit cup is also the stripper name of the mother of a girl named Sally.
WARNING: If you say this to any non-fruit consumer, you will to sentenced to death, you have been warned.
Sally: The earth is flat
Rico: Okay, fruit cup
Sally: No, fruit cup is my mother's name.