'blockhead' originates from English, specifically a compound of 'block' + 'head', where 'block' meant 'a lump or block of wood' and 'head' meant 'head'.

'block' came into English from Middle English 'blok' (ultimately from Old Norse 'blokkr' or Middle Dutch 'bloc'), and the compound 'blockhead' emerged in Early Modern English (around the 16th century) as an insulting term.

Initially it described a head like a block (i.e., thick or wooden-headed); over time it evolved into the general insulting sense 'a stupid person', which is the current meaning.