-ioussuffix

a compound suffix, consisting of the suffix -ous, added to an i which is part of another suffix, representing Latin -iōsus, French -ieux, with sense ‘characterized by, full of’. Found in Latin in adjectives formed from derivative nouns in -ia, -ies, -ius, -ium, as invidiōsus invidious, perniciōsus pernicious, ēbriōsus drunken, ebrious, odiōsus odious; by false analogy in cūriōsus curious (from cūra): see -ous suffix. Also in adjectives belonging to nouns in -io, -iōn-em, as ambit-iōs-us (from ambit-iōn-em) ambitious; so captiōsus captious, factiōsus factious, obliviōsus oblivious, religiōsus scrupulous, religious, seditiōsus seditious, suspiciōsus suspicious, etc. By analogical extension from these, there is a tendency in English to form an adjective in -ious beside any noun in -ion, esp. those in -tion, -cion, -sion, e.g. rebell-ion, -ious, caution, -ious, infection, -ious, contradictious, deceptious, disputatious, dissentious (for -sious), ignitious. Adjectives in -ious are also formed in English by adding -ous to the stem of Latin adjectives in -i-us, e.g. Latin vari-us various; also in -itious from Latin -īci-us, as adventitious: see -itious suffix¹, and -ous suffix.