To work steadily at something, often traveling back and forth regularly; or a layer or thickness of material.
From Old French 'plier' (to fold or bend), from Latin 'plicare.' The noun form comes from the idea of folding or layering. The verb meaning 'to work steadily' evolved metaphorically from the idea of continuous bending motion.
When you hear 'three-ply' on toilet paper, you're hearing an old word that means 'three folds'—the same word that describes someone who regularly plies the same route, because both involve repeated, consistent folding or traveling!
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