White girl equivalent of fuck you and all that you stand for. The highest insult possible from a white high schoolteenager, used only in cases of extreme distaste. Different levels of hate are represented by these variations: K, K Cya, Kaaaaay
"So, I heard you hooked up with my ex"
"Yeah, and..."
"K Bye"
The term can also be used to indicate extreme decisiveness or self-confidence. In this sense it is not antagonistic, but rather a way of injecting a greater sense of assertiveness and urgency into a statement.
Waiter: "What can I get you?"
You: "I'm gonna have the steak tartar and frites, thanks, bye'"
Like "before I/we forget," "by the bye" means the speaker (or writer) is going to change the subject. It comes from "by" (meaning "near") "the bye" (the closing, where we say "goodbye.") So it means, "before we say bye, let me also tell you...."
"... and that's all I heard about Brenda and Eddie. By the bye, have you heard from Brenda's friend Billy?"