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1590s, "side of a ship" (technically, "the side of a ship above the water, between the bow and the quarter"), from broad (adj.) + side (n.); thus "the artillery on one side of a ship all fired off at once" (1590s, with figurative extensions). Two words until late 18c. Of things other than ships, 1630s. But oldest-recorded sense in English is "sheet of paper printed only on one side" (1570s).