-ous Also -eous, -ious, and -ulous.
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.
See also -ose1.