1. You've 'done already known' about something. In other words, you should know, or you do know.
2. An expletive to celebrate something, in the same way you'd say 'get in'.
3. Used incorrectly by many who think it means 'you don't know'.
Skepta: Dunkno the myspace (you already know about the myspace)
2. When I score a sick goal on FIFA: Dunkno! Finessed that to shit!
Another reason is because they think it's cool to use slang and miss-pronounce everything.
Peter: "Yo comin' to da party ta-night?!"
Jase: " Dun kno mann,think imma be somewhier!"
Hailey: "Hey, do you know where I put my keys? I had them just a-"
Carlos: "Dun know..(walks away)"