mid-13c., "abundance; as much as one could desire; an ample supply," from Old French plentee, earlier plentet "abundance, profusion" (12c., Modern French dialectal plenté), from Latin plenitatem (nominative plenitas) "fullness," from plenus "full, filled, greatly crowded; stout, pregnant; abundant, abounding; complete," from PIE root *pele- (1) "to fill."
From early 14c. as "a large amount, a great deal." The meaning "condition of general abundance" is from late 14c. The colloquial adverb meaning "very much" is first attested 1842. Middle English had parallel formation plenteth, from the older Old French form of the word.
Two square feet, or four at most, in one corner of the frame, will give you mustard and cress a plenty for salads, if you take care to make repeated sowings in proper time. [William Cobbett, "The English Gardener," 1829]
But perhaps older, depending how some uses of aplenty or a plenty are read.
c. 1300, plenteivous, "fertile, fruitful, prolific," from Old French plentivos, plentiveus "fertile, rich" (early 13c.), from plentif "abundant," from plentee "abundance" (see plenty). From late 14c. as "abundant, plentiful, copious." Modern form by late 14c. Related: Plenteously; plenteousness.
plenteous, -iful. As with other pairs in -eous & -iful (e.g. from bounty, beauty, duty, pity), the meaning of the two is the same, but the -eous word is the less common & therefore better suited to the needs of poetry & exalted prose ; for these it should be reserved. [Fowler, 1926]
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."
late 14c., "make a copy of, duplicate" (a text or document), from Old French copier (14c.) and directly from Medieval Latin copiare "to transcribe," originally "to write in plenty," from Latin copia "plenty" (see copy (n.)). Hence, "to write an original text many times."
Figurative sense of "to imitate, to follow as an example" is attested from 1640s. Of computer data, by 1953. Meaning "send a copy (of a letter, later e-mail, etc.) to a third party" is attested by 1983. Related: Copied; copying.
"(self-)important person," 1912, short for Chinook jargon high muck-a-muck, literally "plenty of food" from muckamuck "food" (1847), which is of unknown origin. Also mucky-muck; muckety-muck.