-ous

Also ‑eous, ‑ious, and ‑ulous.

Forming adjectives.

Latin ‑osus.

The ‑ous ending is extremely common and is a standard way of forming adjectives, either from words of French or Latin origin (in the latter case usually from nouns ending in ‑us), or from native English nouns. Examples include cancerous, dangerous, generous, libellous, mountainous, ominous, poisonous, thunderous, and wondrous.

The form ‑ious is a variant from Latin ‑iosus, often via French ‑ieux (cautious, curious, delirious, mysterious, precious, spacious, vivacious); examples in ‑eous are from Latin words ending in ‑eus (aqueous, calcareous, extraneous, instantaneous, simultaneous, vitreous). Those in ‑ulous are usually from Latin words ending in ‑ulosus or ‑ulus (fabulous, miraculous, populous, ridiculous).

The ‑ous ending frequently appears in compound suffixes, separate entries for which are at ‑aceous, ‑androus, ‑ferous, ‑gerous, ‑gynous, and ‑parous. See also ‑cephalic (for ‑cephalous), ‑mer (for ‑merous), ‑phagy (for ‑phagous), ‑phile (for ‑philous), ‑phore (for ‑phorous), and ‑vore (for ‑vorous).

In chemistry, ‑ous specifically denotes an element having a lower combining power: cuprous, ferrous, nitrous, sulphurous. In such cases, the higher valency is marked by ‑ic.