word-forming element meaning "many, much, multi-, one or more," from Greek polys "much" (plural polloi), from PIE root *pele- (1) "to fill," with derivatives referring to multitudinousness or abundance. Equivalent to Latin multi-, it is properly used in compounds only with words of Greek origin. In chemical names, usually indicating a compound with a large number of atoms or molecules of the same kind (such as polymer).
1610s, "deductive reasoning," from Latin synthesis "collection, set, suit of clothes, composition (of a medication)," from Greek synthesis "composition, a putting together," from syntithenai "put together, combine," from syn- "together" (see syn-) + tithenai "to put, to place," from reduplicated form of PIE root *dhe- "to set, put." From 1733 as "a combination of parts into a whole." Earlier borrowed in Middle English as sintecis (mid-15c.). Plural syntheses.
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<a href="https://www.etymonline.com/word/polysynthesis">Etymology of polysynthesis by etymonline</a>
Harper, D. (n.d.). Etymology of polysynthesis. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/polysynthesis