If you refer to someone as a numbskull, you mean that they are very stupid.

What does the numbskull mean?

1 : a dull or stupid person : dunce. 2 : a thick or muddled head.

Where did the saying numbskull come from?

numbskull (n.)

"dull-witted or stupid person," 1717, numskull, from num, old spelling of numb (adj.) + skull. Numskulled (adj.) is attested from 1706.