A nail or spike that costs or costs forty pence, or is classified by a historical penny-weight system used in carpentry and construction.
From 'forty' plus 'penny', a British currency unit. In carpentry, nails were historically classified by the number you could buy for a penny (when currency was more uniform), so a '40-penny nail' was a specific size classification.
The old 'd' system for nails (where '8d', '10d', '16d' represented different sizes) came from 'penny weight'—carpenters still use these abbreviations today even though nobody remembers the actual penny-buying system, making it one of language's fossilized relic systems!
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