The Collaborative International Dictionary of English v.0.48:

Inadequate \In*ad"e*quate\, a. [Pref. in- not + adequate: cf. F.
   inad['e]quat.]
   Not adequate; unequal to the purpose; insufficient;
   deficient; as, inadequate resources, power, conceptions,
   representations, etc. --Dryden. -- In*ad"e*quate*ly, adv.
   -- In*ad"e*quate*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inadequate
    adj 1: lacking the requisite qualities or resources to meet a
           task; "inadequate training"; "the staff was inadequate";
           "she was unequal to the task" [syn: inadequate,
           unequal] [ant: adequate, equal]
    2: not sufficient to meet a need; "an inadequate income"; "a
       poor salary"; "money is short"; "on short rations"; "food is
       in short supply"; "short on experience" [syn: inadequate,
       poor, short]

Moby Thesaurus II by Grady Ward, 1.0:

144 Moby Thesaurus words for "inadequate":
   adulterated, arrested, asymmetric, barren, base, blemished,
   bootless, callow, counterproductive, damaged, defective, deficient,
   disappointing, disheartening, disparate, displeasing,
   disproportionate, dissatisfactory, effete, emasculate, embryonic,
   empty, erroneous, etiolated, failing, fallible, fatuitous, fatuous,
   faulty, feckless, forceless, found wanting, fruitless, futile,
   half-assed, hypoplastic, ill-matched, ill-qualified, ill-sorted,
   immature, impaired, imperfect, impotent, imprecise, impure,
   in arrear, in arrears, in default, in short supply, inaccurate,
   inane, inappreciable, incapable, incommensurate, incompetent,
   incomplete, inconsiderable, ineffective, ineffectual,
   inefficacious, inexact, infant, inferior, inoperative,
   insufficient, invalid, invertebrate, irregular, lacking, little,
   makeshift, maladjusted, maladroit, mean, mediocre, minus,
   mismatched, missing, mixed, needing, not comparable, not enough,
   not equal to, not good enough, not in it, not perfect, not up to,
   nugacious, nugatory, odd, of no force, off, out of it,
   out of proportion, part, partial, patchy, petty, scant, scanty,
   scarce, scrappy, scrimpy, shabby, short, short of, shy, sketchy,
   skew, skewed, skimpy, small, spineless, sterile, too little,
   trivial, unable, unadapted, unadjusted, unavailing, underdeveloped,
   undeveloped, unequal, unequal to, unequipped, uneven, unfinished,
   unfit, unfitted, unfulfilling, ungratifying, unperfected,
   unqualified, unreached, unsatisfactory, unsatisfying, unskillful,
   unsound, unsufficing, unthorough, useless, vain, wanting, weak