caisson

noun

1. a structure used in underwater work, consisting of an airtight chamber, open at the bottom and containing air under sufficient pressure to exclude the water.
2. a boatlike structure used as a gate for a dock or the like.
3. Nautical.
   1. Also called camel, pontoon.a float for raising a sunken vessel, sunk beside the vessel, made fast to it, and then pumped out to make it buoyant.
   2. a watertight structure built against a damaged area of a hull to render the hull watertight; cofferdam.
4. a two-wheeled wagon, used for carrying artillery ammunition.
5. an ammunition chest.
6. a wooden chest containing bombs or explosives, used formerly as a mine.
7. Architecture. coffer(def 4).

noun

1. a watertight chamber open at the bottom and containing air under pressure, used to carry out construction work under water
2. a similar unpressurized chamber
3. a watertight float filled with air, used to raise sunken shipsSee also camel (def. 2)
4. a watertight structure placed across the entrance of a basin, dry dock, etc, to exclude water from it
5. 1. a box containing explosives, formerly used as a mine
   2. an ammunition chest
   3. a two-wheeled vehicle containing an ammunition chest
6. another name for coffer (def. 3)

n.

1704, from French caisson “ammunition wagon, box, crate,” from Middle French caisson “large box” (16c.), from Italian cassone, augmentative form of cassa “a chest,” from Latin capsa “a box” (see case (n.2)).