What is "coffer dam"

Coffer \Cof"fer\ (?; 115), n. [OF. cofre, F. coffre, L. cophinus basket, fr. Gr. ?. Cf. Coffin, n.]

1. A casket, chest, or trunk; especially, one used for keeping money or other valuables.
   --Chaucer.

   In ivory coffers I have stuffed my crowns.
   --Shak.

2. Fig.: Treasure or funds; -- usually in the plural.

   He would discharge it without any burden to the queen's coffers, for honor sake.
   --Bacon.

   Hold, here is half my coffer.
   --Shak.

3. (Arch.) A panel deeply recessed in the ceiling of a vault, dome, or portico; a caisson.

4. (Fort.) A trench dug in the bottom of a dry moat, and extending across it, to enable the besieged to defend it by a raking fire.

5. The chamber of a canal lock; also, a caisson or a cofferdam.

   Coffer dam. (Engin.) See Cofferdam, in the Vocabulary.

   Coffer fish. (Zo["o]l.) See Cowfish.