-ous

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Possessing; full of; characterized by: joyous.
2. n. Having a valence lower than that of a specified element in compounds or ions named with adjectives ending in -ic: ferrous.

Century Dictionary and Cyclopedia

1. A suffix of Latin origin, forming, from nouns, adjectives denoting fullness or abundance, or sometimes merely the presence, of the thing or quality expressed by the noun, as in callous, famous, generous, odious, religious, sumptuous, vicious, etc. (see etymology). Many modern English adjectives taken directly from the Latin have -ose, as jocose, verbose, with or without an equivalent form in -ous, as herbose herbous, onerose onerous, vinose vinous, spicous spicose, etc., the form in -ose being especially common in botanical terms. By reason of the agreement in the terminal pronunciation of English adjectives in -ous and the English pronunciation of Latin adjectives in -us (in Latin a mere nominative termination), many such adjectives in -us have been transferred into English with the accommodated termination -ous, as anxious, conspicuous, devious, obvious, previous, serious, etc., from Latin anxius, conspicuus, devius, obvius, prævius, serius, etc. So with Latin or New Latin adjectives in -us from Greek -ος, as in acephalous, etc. The suffix -ous is felt as an English formative only when a noun accompanies the adjective, as in famous, odious, religious, ambitious, etc., associated with the nouns fame, odium, religion, ambition, etc. It is sometimes used (as also -ose), as an English formative, attached to words of non-Latin origin, as in quartzous or quartzose, etc.
2. n. In chem., a suffix used to denote the presence in a compound of a relatively electronegative constituent in smaller proportion than in the corresponding compound of which the name bears the suffix -ic. In each case the suffix is attached to the name of the relatively electropositive constituent, as ferrous oxid (FeO) and ferric oxid (Fe₂O₃), stannous chlorid (SnCl₂) and stannic chlorid (SnCl₄).

Wiktionary

1. n. Used to form adjectives from nouns.
2. n. chemistry Used in chemical nomenclature to name chemical compounds in which a specified chemical element has a lower oxidation number than in the equivalent compound whose name ends in the suffix -ic. For example sulphuric acid (H2SO4) has more oxygen atoms per molecule than sulphurous acid (H2SO3). See Inorganic nomenclature.

GNU Webster's 1913

1. n. 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. n. (Chem.) A suffix denoting that the element indicated by the name bearing it, has a valence lower than that denoted by the termination -ic