Dunderhead

dunderhead... as in a foolish, stupid person; dunce; blockhead... Can anyone help with the origin of "dunder?"

American Heritage Dict.:

"Perhaps 'one stunned by a thunderstroke': Dutch 'donder,' thunder, from Middle Dutch."

dunner, dunder 1 make a noise like thunder, rumble, bang. 2 move quickly and noisily.
donnert dull, stupid, witless, especially in old age.
From _The Pocket Scots Dictionary_