I learned, when doing Renaissance costuming, that the legs of hosen (the ancestors of pants/trousers/etc.) were separate, and laced to the doublet. Eventually, as doublets shortened, the hosen became breeches and the lacings were reduced to the fly front, but they’ve remained a “pair.”
Of course, the fact that that now extends to “pair of undies” is just patently silly, but what can you do?
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