'thickheaded' originates from English, specifically the adjective 'thick' and the noun 'head', where 'thick' ultimately comes from Old English 'þicce' meaning 'dense' and 'head' comes from Old English 'heafod' meaning 'head'.

'thick' changed from Old English 'þicce' to Middle English forms such as 'thikke', and 'head' from Old English 'heafod' to Middle English 'hed'; the compound adjective 'thick-headed' developed in Early Modern English as a figurative description of mental qualities.

Initially it could be taken more literally as 'having a physically thick head' or 'heavy-headed'; over time the phrase became chiefly figurative, meaning 'dull, slow to understand' and that figurative sense is dominant today.