'Poly-,' 'plenty,' and 'full' are distant cousins — all from PIE *pelh1- (to fill, abundance).
A prefix meaning 'many,' 'much,' or 'several,' derived from Greek and used to form words indicating plurality, multiplicity, or abundance.
From Greek πολύς (polýs, "much, many"), from PIE *pelh₁-u- ("much, many, full"), an adjectival formation from the root *pelh₁- ("to fill"). This root is one of the most prolific in Indo-European, generating vocabulary across nearly every daughter language: Latin plēnus ("full"), plūs ("more"), plūrēs ("several"), plēbs ("the masses, the many"); Sanskrit purú- ("much, many"), pūrṇá- ("full"); Lithuanian pilnas ("full"); Old English full and folk ("the many people"); Gothic filu ("much"); Old Irish lán ("full," with l from *pl-); and Armenian lի (li, "full"). The Greek form πολύς became extraordinarily productive as a combining prefix in both
A 'polymath' is literally 'a person who has learned many things' — from 'polýs' (many) + 'máthēma' (learning, knowledge). The same root 'máthēma' gives us 'mathematics,' which literally means 'things that are learned.' So a polymath is someone who has absorbed many 'mathematics' in the broadest sense — many fields