thickskull

plural thickskulls

1. archaicA dullard; a blockhead.

   And how often do we hear persons stigmatized as 'numskulls' and 'thickskulls' and 'blockheads' — though why they should be block-heads, I know not […]

   1859, William Swinton, Rambles Among Words: Their Poetry, History and Wisdom

   1 more example

   the dullwitted pair began to think of redeeming their turbans, and in spite of the entreaties of Agib, being very obstinate, as such thickskulls usually are, they went forth on that errand.

   c. 1828, Thomas Hood, The Three Brothers