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.
