In carpentry, referring to a nail or fastener that costs or weighs approximately fifty pennies, or is of a standard size denoted by that historical pricing.
From fifty plus penny. In old English nail sizing, prices were labeled by the penny—a 50-penny nail was a specific, larger size useful for heavy construction work.
Old nail sizing by penny is a quirky historical system: a 16-penny nail (written '16d') isn't 16 times bigger than a 1-penny nail—the system is logarithmic and utterly illogical by modern standards.
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