The Collaborative International Dictionary of English v.0.48:

Deficient \De*fi"cient\, a. [L. deficiens, -entis, p. pr. of
   deficere to be wanting. See Defect.]
   Wanting, to make up completeness; wanting, as regards a
   requirement; not sufficient; inadequate; defective;
   imperfect; incomplete; lacking; as, deficient parts;
   deficient estate; deficient strength; deficient in judgment.
   [1913 Webster]

         The style was indeed deficient in ease and variety.
                                                  --Macaulay.
   [1913 Webster]

   Deficient number. (Arith.) See under Abundant. --
      De*fi"cient-ly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

deficient
    adj 1: inadequate in amount or degree; "a deficient education";
           "deficient in common sense"; "lacking in stamina";
           "tested and found wanting" [syn: deficient,
           lacking(p), wanting(p)]
    2: of a quantity not able to fulfill a need or requirement;
       "insufficient funds" [syn: insufficient, deficient] [ant:
       sufficient]
    3: falling short of some prescribed norm; "substandard housing"
       [syn: deficient, inferior, substandard]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "deficient":
   adulterated, amiss, arrested, bad, base, blemished, botched,
   bungling, callow, clumsy, damaged, defective, embryonic, erroneous,
   failing, fallible, faulty, flawed, found wanting, half-assed,
   haphazard, hit-and-miss, hit-or-miss, hypoplastic, immature,
   impaired, imperfect, imprecise, impure, in arrear, in arrears,
   in default, in short supply, in want of, inaccurate, inadequate,
   incompetent, incomplete, inexact, infant, inferior, infrequent,
   injured, insufficient, lacking, little, makeshift, maladroit,
   marred, mean, mediocre, messy, minus, missing, mixed, needing,
   not comparable, not enough, not in it, not perfect, off, out of it,
   part, partial, patchy, petty, promiscuous, rare, scant, scanty,
   scarce, scrappy, scrimpy, shabby, short, short of, shy, sketchy,
   skimpy, slipshod, slipshoddy, sloppy, slovenly, sluttish, small,
   too little, trivial, uncommon, underdeveloped, undeveloped,
   unequal to, uneven, unfinished, unperfected, unqualified,
   unreached, unsatisfactory, unsatisfying, unskillful, unsound,
   unsufficing, unthorough, untidy, wanting