To work steadily at something, or to travel regularly back and forth on a route, or to persistently push or urge.
From 'ply,' shortened from Old French 'plier' meaning 'to bend' or 'to fold.' The sense shifted to mean 'to work steadily' and 'to travel a route,' especially in nautical contexts from the 17th century onward.
Ferries and water taxis 'ply' their routes—the word is still most common in maritime contexts where it describes ships that work the same route repeatedly, day after day, coming and going like steady breathing.
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