A saucepan containing hot water into which a smaller saucepan is plunged, either to keep it hot, or that it may boil without burning. A glue pot is a good example. Mons. Bouillet says, “Ainsi appelé du nom de l'inventeur ” (Balneum Mariæ). But derivations from proper names require authentication.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894