aplenty (adj.)

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.