To call someone bird brained is to say they are foolish, scatty, or lacking in sense, as if their skull housed nothing bigger than the brain of a sparrow. It is a mild, everyday insult rather than a harsh one, usually aimed at forgetfulness, impracticality, or a general flightiness of thought rather than real stupidity. The word can be used as a noun too, a “bird brain” being the person rather than the quality, and both forms trade on the same old assumption that a creature as small as a bird cannot have much going on inside its head.
What’s the origin of the phrase ‘bird brained’?
The idea behind the phrase is considerably older than the compound word itself. English already had “bird-witted” for someone flighty and empty-headed, and the OED’s earliest evidence for that word is from 1605, in the writing of Francis Bacon, the philosopher and lord chancellor. Bacon’s use shows that comparing a scatterbrained person to a bird was established well before the Stuart period was out, long predating the specific “brain” version.
Exactly when “bird brain” and “bird brained” themselves entered common use is harder to pin down with certainty. Members of the American Dialect Society mailing list, searching old newspaper archives, turned up an insulting use of “bird brain” in the Goshen Democrat of Indiana from 1864, where a writer sneered at an opponent “employing meanwhile, his little chipping bird brain” for attacking fellow Democrats. That find sits outside the OED’s own citation record and comes from informal research rather than a dictionary, so it is best treated as a promising early lead rather than a confirmed first use. It does suggest, though, that the insult was already circulating in American English in the mid nineteenth century, feeding off the same centuries-old assumption behind “bird-witted” that a bird’s brain is too small to hold much sense.