a suffix forming adjectives that correspond to nouns ending in the suffixes -acity and -acy: audacious; fallacious.
Origin of -acious
< Latin -āci- (stem of -āx) adj. suffix + -ous
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Word Origin and History for -acious
adjectival word-forming element meaning "given to, inclined to, abounding in," from Latin -aci- (nominative -ax), noun ending used with verbal stems, + -ous.
Online Etymology Dictionary, © 2010 Douglas Harper