1. A phrase used to convey a realization, often mid-sentence. I wasn't going to go out with you guys tonight, but hey, you know what, I'm in!

2. A rhetorical question posed before a statement one considers to be the blunt truth, often during an argument or heated exchange. You know what, John? I think you never really loved me.

3. A statement asserting that the person to whom one is speaking does in fact know something that they claim not to. A: "What is the matter with you?" B: "You know what!"