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]