-ous \-ous\ [OF. -ous, us, -os, F. -eux, fr. L. -osus, and -us. Cf. -ose.]

1. An adjective suffix meaning full of, abounding in, having, possessing the qualities of, like; as in gracious, abounding in grace; arduous, full of ardor; bulbous, having bulbs, bulblike; riotous, poisonous, piteous, joyous, etc.

2. (Chem.) A suffix denoting that the element indicated by the name bearing it, has a valence lower than that denoted by the termination -ic; as, nitrous, sulphurous, etc., as contrasted with nitric, sulphuric, etc.

Source: Webster's Revised Unabridged Dictionary (1913)