-ous

CIDE DICTIONARY

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

•  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.  [1913 Webster]
•  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, mercurous, ferrous, sulphurous, etc., as contrasted with nitric, mercuric, ferric, sulphuric, etc.  [1913 Webster]

OXFORD DICTIONARY

-ous, suffix.
1 forming adjectives meaning 'abounding in, characterized by, of the nature of' (envious; glorious; mountainous; poisonous).
2 Chem. denoting a state of lower valence than the corresponding word in -ic (ferrous).

Derivative

-ously suffix forming adverbs. -ousness suffix forming nouns.

Etymology

from or after AF -ous, OF -eus, f. L -osus