A person who is stupid, slow to understand, or boring; someone lacking intelligence or wit.
From the adjective 'dull' (not sharp, not bright, not interesting) + the suffix '-ard' (a derogatory label, as in 'drunkard' and 'coward'). This insult has been used in English since at least the 1500s.
The suffix '-ard' in dullard, drunkard, and coward originally meant 'prone to' or 'full of'—so a dullard is someone 'full of dullness,' which sounds silly but was meant as a serious insult!
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