have a screw loose

Idioms & Phrases

have a screw loose

Be mentally unstable or eccentric, as in Anyone who approves that purchase must have a screw loose. This term likens a mental weakness to a machine in which a part is not securely fastened. An antonym is have one's head screwed on right; for example, She's very capable; she has her head screwed on right. [Slang; early 1800s]

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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