1. a suffix forming adjectives that have the general sense “possessing, full of ” a given quality (covetous; glorious; nervous; wondrous); -ous and its variant -ious have often been used to Anglicize Latin adjectives with terminations that cannot be directly adapted into English (atrocious; contiguous; garrulous; obvious; stupendous). As an adjective-forming suffix of neutral value, it regularly Anglicizes Greek and Latin adjectives derived without suffix from nouns and verbs; many such formations are productive combining forms in English, sometimes with a corresponding nominal combining form that has no suffix; cf. -fer, -ferous; -phore, -phorous; -pter, -pterous; -vore, -vorous.
2. a suffix forming adjectival correspondents to the names of chemical elements; specialized, in opposition to like adjectives ending in -ic, to mean the lower of two possible valences (stannous chloride, SnCl2, and stannic chloride SnCl4).
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.