Small sharp nails with flat heads used for fastening light materials. Plural of tack, also referring to courses of action or approaches to a problem.
From Old French 'taque' meaning a nail or fastener, possibly from Germanic origins. The nautical sense of 'tack' (changing direction) developed from the idea of securing sails with tacks, leading to the figurative meaning of changing approach or strategy.
The phrase 'sharp as tacks' ironically refers to mental acuity rather than physical sharpness, since tacks are actually quite dull compared to needles or pins. In sailing, 'tacking' involves turning the bow through the wind, which gave us the expression 'taking a different tack' for changing strategy.
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