Describing something that costs or is worth eight pence, or referring to a size of nail that is eight pennies in weight.
From 'eight' plus 'penny', referring to the old British currency system. In nails, the 'd' abbreviation (from Latin 'denarius') was used: 8d meant eightpenny.
Hardware stores still list nails as 'eightpenny' or '8d' because this medieval pricing system stuck around—it's one of the few places where ancient weights-and-measures still survive!
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