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Wood hardens with age. After a period of time, you can't drive nails in some types. A carpenter might call such wood "harder than nails".

The term could be applied to many other materials that harden with age, often as an exaggeration; often food.

hard, stale, inedible
Those left-over biscuits are harder than nails.
by caloo_calay December 01, 2013
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