"flat-bottomed boat" (especially, in military engineering, one to support a temporary bridge over a river), 1670s, from French pontoon, from Old French ponton (14c.) "bridge, drawbridge, boat-bridge; flat-bottomed boat," from Latin pontonem (nominative ponto) "flat-bottomed boat," from pons "bridge" (see pons). Pontoon bridge "roadway supported on pontoons" is recorded by 1778. Related: Pontooneer (n.).
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