mid-15c., splendoure, "radiance, brilliance, source of magnificence or excellence," from Old French esplendour, Anglo-French esplendour (Old French splendeur, splendor, 12c.) and directly from Latin splendor "brilliance, brightness," from splendere "be bright, shine" (see splendid).
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."
<a href="https://www.etymonline.com/word/splendorous">Etymology of splendorous by etymonline</a>
Harper, D. (n.d.). Etymology of splendorous. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/splendorous