word-forming element, before vowels oct-, from combining form of Latin octo "eight," from PIE root *octo(u)- "eight" (see eight). Words made from Greek elements or derived from Greek typically are octa-.
word-forming element, Latin -plus "-fold." Watkins derives it from *-plo-, combining form of PIE root *pel- (2) "to fold" and makes it cognate with Old English -feald, Greek -paltos, -plos. But de Vaan connects it to PIE root *pele- (1) that yielded words meaning "much, many, more" and is the source of poly-.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit putah "fold, pocket;" Albanian pale "fold;" Middle Irish alt "a joint;" Lithuanian pelti "to plait;" Old English faldan "to fold, wrap up, furl."
<a href="https://www.etymonline.com/word/octuple">Etymology of octuple by etymonline</a>
Harper, D. (n.d.). Etymology of octuple. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/octuple