Foolish or stupid; lacking intelligence or good judgment.
Combines 'bird' with 'brained,' emerging in the 19th century based on the folk belief that birds have small brains and therefore little intelligence. The phrase became a common insult to describe someone as scatter-brained or silly.
Birds actually have proportionally larger brains relative to body size than many mammals! This insult is scientifically backwards—crows and parrots are among the smartest animals on Earth, yet 'birdbrained' stuck around as an insult anyway, showing how language preserves old myths even when they're completely wrong.
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