A pontoon is a flat-bottomed floating structure, often a hollow metal or plastic tube, that is used to support a bridge, dock, or boat. Boats with pontoons are very stable and sit high on the water.
From French *ponton* “flat-bottomed boat, floating bridge,” from Latin *pōns* (genitive *pontis*) “bridge.” The word originally referred to floating bridges used in military campaigns.
The link to *bridge* is still visible in the word: pontoons are like portable floating roads. Modern party boats on lakes are using the same basic idea as ancient armies crossing rivers.
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