"containing tin," 1790, from Modern Latin stannum, from Late Latin stannum "tin" (earlier "alloy of silver and lead"), a scribal alteration of Latin stagnum, probably from a Celtic source (compare Irish stan "tin," Cornish and Breton sten, Welsh ystaen). The Latin word is the source of Italian stagno, French étain, Spanish estaño "tin."
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/stannous">Etymology of stannous by etymonline</a>
Harper, D. (n.d.). Etymology of stannous. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/stannous