Whatever is the term that teenagers use, when they disagree with something, or don't care about something what so ever. It's a type of hormone that teens get, when they're real snotty, and cares about nothing but themselves.
Sometimes when a teenager's parent tells them: We're going on a trip to Mexico! Aren't you excited? Their immediate response is, Whatever. It kind of means that they weren't really listening, and they are too caught up with their friends, texting, playing computer games, or something or other, that they don't care what their parent has to say.
It's like when a teen is not listening, and someone speaks to them, but they're too concentrated on something else, that they agree to something that maybe they don't even want to do. This word can get used in pretty snotty ways sometimes.
You don't agree with the above definition? Well, I don't care. Whatever.