lacus

Lacus, lacus, m., fourth declension, with dative and ablative plural lacubus rather than the lacibus you would expect. It is a lake, but the word covers any basin that holds liquid: the vat in a winery, the tank in a bath house, the trough beneath a fountain, and a pit dug in the ground.

That last sense matters for reading the Vulgate. Daniel goes into a lacus leonum, which is a pit and not a lake, and translating it literally produces a much wetter and much stranger story. Lacuna, a hollow or gap, is the diminutive, and it gives English the gap in a damaged manuscript. English lake came in from French lac and blended with a native word of similar sound; loch and lough are Celtic cousins of the same ancient root.