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.