Where does the word “Cofferdam” come from and What does Cofferdam mean?

Though in our impious youth one was considered as very clever who could work this in as, “Let her cofferdam head off,” it really has no connection with an oath.

A coffer is merely a tight chest, as for the storing of valuables.

By extension, it denotes a watertight box or caisson.

And a cofferdam is nothing more than a series of such caissons so constructed as to become a dam for holding out water.