Webster’s Revised Unabridged Dictionary · 1913
Manganous
Man′ga·nous (măn̲′gȧ·nŭs), a. (Chem.) Of, pertaining to, designating, those compounds of manganese in which the element has a lower valence as contrasted with manganic compounds; as, manganous oxide.
Manganous acid, a hypothetical compound analogous to sulphurous acid, and forming the so-called manganites.