I. ˈplen-tē noun
Etymology: Middle English plente, from Anglo-French plenté, from Late Latin plenitat-, plenitas, from Latin, fullness, from plenus full — more at full
Date: 13th century
1.
a. : a full or more than adequate amount or supply
had plenty of time to finish the job
b. : a large number or amount
in plenty of trouble
2. : the quality or state of being copious : plentifulness
II. adjective
Date: 14th century
1. : plentiful in amount, number, or supply
if reasons were as plenty as blackberries — Shakespeare
2. : ample
plenty work to be done — Time
Usage:
Many commentators object to use of sense 2 in writing; it appears to be limited chiefly to spoken English. Sense 1 is literary but is no longer in common use.
III. adverb
Date: 1842
: more than sufficiently : to a considerable degree
the nights were plenty cold — F. B. Gipson
Usage:
Many handbooks advise avoiding the adverb plenty in writing; “use very, quite, or a more precise word,” they advise. Actually plenty is often a more precise word than its recommended replacements; very, fully, or quite will not work as well in these typical quotations
it's already plenty hot for us in the kitchen without some dolt opening the oven — C. H. Bridges
may not be rising quite as rapidly as other health costs, but it is going up plenty fast — Changing Times
It is not used in more formal writing.