The Collaborative International Dictionary of English v.0.48:

Plenary \Ple"na*ry\, a. [LL. plenarius, fr. L. plenus full. See
   Plenty.]
   Full; entire; complete; absolute; as, a plenary license;
   plenary authority.
   [1913 Webster]

         A treatise on a subject should be plenary or full. --I.
                                                  Watts.
   [1913 Webster]

   Plenary indulgence (R. C. Ch.), an entire remission of
      temporal punishment due to, or canonical penance for, all
      sins.

   Plenary inspiration. (Theol.) See under Inspiration.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plenary \Ple"na*ry\, n. (Law)
   Decisive procedure. [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

plenary
    adj 1: full in all respects; "a plenary session of the
           legislature"; "a diplomat with plenary powers"

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "plenary":
   SRO, absolute, brimful, brimming, bulging, bursting, capacity,
   chock-full, chuck-full, comprehensive, congested, consequential,
   considerable, cram-full, crammed, deep, exhaustive, farci, filled,
   flush, full, full to bursting, grand, grave, great, heavy,
   illimitable, intense, irresistible, jam-packed, limitless, main,
   maximum, mighty, no strings, open, overfull, overstuffed, packed,
   packed like sardines, perfect, powerful, ready to burst, replete,
   round, satiated, saturated, serious, soaked, standing room only,
   strong, stuffed, surfeited, swollen, topful, total, unbound,
   unbounded, uncircumscribed, unconditional, unconditioned,
   unconfined, unequivocal, unlimited, unmeasured, unqualified,
   unrestricted, wide-open, without strings



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PLENARY. Full, complete.
     2. In the courts of admiralty, and in the English ecclesiastical
courts, causes or suits in respect of the different course of proceeding in
each, are termed plenary or summary. Plenary, or full and formal suits, are
those in which the proceedings must be full and formal: the term summary is
applied to those causes where the proceedings are more succinct and less
formal. Law's Oughton, 41; 2 Chit. Pr. 481.