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A stupid or foolish person, especially one whose thoughts are disjointed or chaotically disorganized. Sometimes hyphenated. These were not the unpredictable acts of some rattlebrain—they were coldly calculated. I don't know how they could let a rattle-brain like her lead this company.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
n. a stupid person. Please try not to be such a rattlebrain! Pay attention to what you are doing.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.