mod. stupid; witless. : George is just soft in the head. He'll never get away with his little plan.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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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.
