a stupid or slow-witted person; blockhead; numbskull.
The Neat FM presenter also said that Prince Tsegah is a dunderhead, who does not deserve a minute of his time. (Ola Michael replies The Don, says he does not think with his head, The Guardian, February 2021)
Later, she called the cops on her boyfriend again; this time the dunderhead copped an aggravating attitude with the officers. (Larry Hobbs, CrimeScene, 7.03, The Brunswick News, July 2021)
I never saw such a dunderhead; can't you understand anything at all? (Mark Twain, A Connecticut Yankee in King Arthur's Court)
I’d like to say that this coat can only be appreciated by someone with a sharpened aesthetic sense - not a dunderhead like you! (Doctor Who, 'The One Doctor' [audio drama] 2001)
1620s, from head; the first element is obscure, perhaps from Middle Dutch doner, donder 'to thunder' (compare blunderbuss). Dunder also was a native dialectal variant of thunder. In the same sense were dunder-whelp (1620s); dunderpate (1754); dunderpoll (1801). (Online Etymology Dictionary)