A foolish or stupid person; someone who acts without thinking.
From cock (rooster) + brain, combining the image of a rooster's small brain with an insult implying stupidity. This compound emerged in Middle English as a derogatory term for an absent-minded or senseless person.
Medieval insults are surprisingly inventive—calling someone a 'cockbrain' relied on the widespread assumption that roosters were dim-witted creatures. The tradition of using animal names for human foolishness (like 'blockhead' or 'birdbrain') shows how much our ancestors thought about what different animals represented.
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