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soft in the head

Mentally deficient; also, silly, foolish. For example, He's nice enough but a bit soft in the head. The soft in this idiom, first recorded in 1775, alludes to a weakness in mental capacity.

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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.