Slurry way of saying "don't know". People either say it like that when they're feeling to lazy to speak properly or don't want to speak at all.
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)"