A stupid or blockheaded person; literally, someone with a head (pate) as thick as a clod of earth.
Compound from 'clod' (lump of earth) and 'pate' (the head or crown of the head, from Middle English 'pate'). This creates a vivid insult suggesting someone's skull is as dense and lumpy as soil.
Medieval insulters loved specific body-part insults—'clodpate' targets the head specifically, implying that's where the stupidity lives, whereas 'blockhead' uses a different wood metaphor to describe the same concept!
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