Dictionary.com: a dunce; blockhead; numbskull. Also called dun·der·pate.
Other forms: dun·der·head·ed, adjective; dun·der·head·ed·ness, noun.
The Online Etymology Dictionary: 1620s; the first element is obscure; perhaps from M.Du. doner, donder “to thunder”.
Mirriam Webster: perhaps from Dutch donder thunder + English head; akin to Old High German thonar thunder — more at thunder. First Known Use: circa 1625
C’mon, let’s use “dunderhead” more; so descriptive, so woody (as in the Monty Python meaning of woody).