Stupid or slow in thinking; lacking mental sharpness or intelligence.
From 'dull' (not sharp, not bright) + 'brained' (having a brain of a certain type, as in 'left-brained'). This compound adjective directly attacks someone's mental capacity by describing their brain as dull.
Dullbrained is a vivid insult that suggests someone's actual brain is literally not sharp—which is funny because our brains don't work like knives, but the metaphor is strong enough to hurt!
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