late 14c., "fresh green color," from Old French verdure "greenness, greenery, green fields, herbs," from verd, variant of vert "green" (12c.), from Latin viridis (source of Spanish, Italian verde), related to virere "be green," of unknown origin. Perhaps ultimately from a root meaning "growing plant" and cognate with Lithuanian veisti "propagate," Old Norse visir "bud, sprout," Old English wise "sprout, stalk, etc." But de Vaan writes that "None of the adduced set of cognates (Lat. 'green', Baltic 'multiply, fruit', Gm. 'sprout, meadow') undoubtedly belong together." Meaning "green plants, vegetation" is attested from c. 1400.
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."