A better, more correct version of 'dunda head.' Dunder brain can be used as a friendly insult, but do not try actually paying somebody out with this word, because you will seem like a complete moron, and a 'dunder brain' yourself.
Can refer to someone who is muddled in the brain, or a generally silly person.
Kayla: "You're a dunda head"
Me: "No, Kayla, it's dunder brain"
Kayla: "You're a retard"