Batardeau

Cannot be auctioned, traded, bazaared, or delivered.
Cannot be obtained as a random reward from the Gobbie Mystery Box Special Dial and similar sources.

Batardeau is French for "cofferdam".

A cofferdam is an enclosure beneath the water constructed to allow water to be displaced by air for the purpose of creating a dry work environment. Commonly used for oil rig construction and repair, bridge and dam work. It is also a kind of military wall built across from fortifications, with sluice gates to control water level on the outside.

Batardeau is also sometimes incorrectly associated in literature with the épée bâtarde, or "bastard sword."