"redundancy in words," 1580s, from Late Latin pleonasmus, from Greek pleonasmos, from pleonazein "to be more than enough, to be superfluous," in grammatical use, "to add superfluously," from combining form of pleon "more" (from PIE root *pele- (1) "to fill"). Not necessarily a flaw in rhetoric and sometimes used effectively for emphasis. As Fowler writes, "The writer who uses [pleonasm] in that way must be judged by whether he does produce his effect & whether the occasion is worthy of it."
The first surplusage the Greekes call Pleonasmus, I call him [too full speech] and is no great fault, as if one should say, I heard it with mine eares, and saw it with mine eyes, as if a man could heare with his heeles, or see with his nose. We our selues ysed this superfluous speech in a verse written of our mistresse, neuertheles, not much to be misliked, for euen a vice sometime being seasonably vsed, hath a pretie grace. [George Puttenham, "The Arte of English Poesie, 1589]
It forms all or part of: accomplish; complete; compliment; comply; depletion; expletive; fele; fill; folk; full (adj.); gefilte fish; hoi polloi; implement; manipulation; nonplus; plebe; plebeian; plebiscite; pleiotropy; Pleistocene; plenary; plenitude; plenty; plenum; plenipotentiary; pleo-; pleonasm; plethora; Pliocene; pluperfect; plural; pluri-; plus; Pollux; poly-; polyamorous; polyandrous; polyclinic; polydactyl; polydipsia; Polydorus; polyethylene; polyglot; polygon; polygraph; polygyny; polyhedron; polyhistor; polymath; polymer; polymorphous; Polynesia; polyp; Polyphemus; polyphony; polysemy; polysyllabic; polytheism; replenish; replete; supply; surplus; volkslied.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit purvi "much," prayah "mostly;" Avestan perena-, Old Persian paru "much;" Greek polys "much, many," plethos "people, multitude, great number," ploutos "wealth;" Latin plus "more," plenus "full;" Lithuanian pilus "full, abundant;" Old Church Slavonic plunu; Gothic filu "much," Old Norse fjöl-, Old English fela, feola "much, many;" Old English folgian; Old Irish lan, Welsh llawn "full;" Old Irish il, Welsh elu "much."