'dull-headed' originates from Modern English as a compound of the adjective 'dull' and the combining form 'headed' (from 'head'), where 'dull' originally meant 'blunt, insensible' and 'head' referred to the head or skull.

'dull' comes from Old English 'dol' (also seen as 'dull'), ultimately from Proto-Germanic *dulaz meaning 'insensible, foolish'; 'head' comes from Old English 'heafod' (Proto-Germanic *haubud-). The compound 'dull-headed' formed in later English by joining these elements to describe someone 'having a dull head' (i.e., slow of mind).

Initially, 'dull' often meant 'stupefied, blunted, not sharp'; over time the sense extended to 'not quick of intellect' and this meaning is preserved in compounds like 'dull-headed'.