plenty (n.)

also a-plenty, "in abundance," by 1829, colloquial when used after the noun, from a- (1) + plenty (n.).

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]