culvert

Nobody actually knows where 'culvert' comes from. Theories include a dialect word, a nod to a forgotten inventor's name, a borrowing from French couvert ('covered'), or an unrecorded Dutch word possibly built from coul- (from French couler, 'to flow') plus vaart ('canal' or 'boat trip').

There isn't a documented shift in meaning to trace here — the mystery lies entirely in where the word came from, not in how its sense changed once it arrived.

One theory ties it to French couvert, 'covered,' though French itself doesn't use couvert this way — it prefers ponceau or buse de drainage for what English calls a culvert.