Lacking good sense or judgment; silly, unwise, or stupid in a way that shows poor decision-making.
From Old French 'fol' (mad, silly) and Latin 'follis' (a bellows or air-filled bag), with the '-ish' suffix meaning 'having qualities of.' The original sense compared foolishness to being 'full of air'—inflated and empty, without substance.
The root of 'foolish' is 'follis,' a Latin word for a bellows or air-filled bag—meaning a fool is literally someone 'full of hot air' with nothing solid inside! This image of empty inflation has survived in language for over 2,000 years.
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