Slang for an eccentric, unstable, or crazy person — someone whose brain is figuratively 'cracked.' Historically used in British and American English from at least the 17th century. The term is not clinical but colloquial, implying someone erratic or unreliable rather than diagnosably ill. Has a slightly quaint, old-fashioned register today.
The village crackbrain spent his evenings on the bench outside the post office, arguing with anyone who stopped.
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(slang) An eccentric or crazy person.
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Regional slang from around the English-speaking world — British, Australian, Irish, Caribbean, Nigerian, Filipino, AAVE, and the hyphenated-English dialects that make the internet sound local.
See all Regional & Other slang on Slangora.