One of the mysteries of the English language finally explained.
1A worm imagined as infesting the brain. Chiefly figurative.
2Zoology and Veterinary Medicine. Any of various parasitic worms that infect the brains of mammals or other vertebrates; especially any nematode of the genera Elaphostrongylus and Parelaphostrongylus (family Protostrongylidae), which parasitize the central nervous system and certain other tissues of deer. Also (without article): worms of this kind.
3Zoology. A liver fluke, Dicrocoelium dendriticum (family Dicrocoelidae), which parasitizes many species of mammal (and occasionally humans), and has a cercarial stage which enters the brain of an intermediate host, the ant, resulting in behavioural changes conducive to transmission to mammals.
Early 17th century; earliest use found in Richard Brathwait (?1588–1673), poet and writer. From brain + worm.
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