Wiktionary
you know what
phr. (context idiomatic English) A phrase used to get someone's attention before announce something.
Wikipedia
You Know What
"You Know What" is the first single off Avant's album Director. It features Lil Wayne and is produced by Jermaine Dupri.
Usage examples of "you know what".
If you've ever donated an aging PC to a friend or relative, you know what happens next: They need help, usually by long-distance.
Do you know what a huge gap there is between the Model B and the last line of sexdroid?