Meaning, "I dislike you strongly," in intelligent language. Used in these situations, not in particular order.
1. Heated argument
2. Child sticking their tongue out (means "fuck you" in toddler")
3. Used when friends embarrass you and you have nothing to say against it
4. When your iPod, computer, or other device freezes up unexpectedly.
You: AWWW, what an adorable little dimpled child you have!! (Condescendingly)
Baby: -Sticks out tongue- Fuck You!!