The Collaborative International Dictionary of English v.0.48:

Inadequacy \In*ad"e*qua*cy\, n. [From Inadequate.]
   The quality or state of being inadequate or insufficient;
   defectiveness; insufficiency; inadequateness.
   [1913 Webster]

         The inadequacy and consequent inefficacy of the alleged
         causes.                                  --Dr. T.
                                                  Dwight.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inadequacy
    n 1: lack of an adequate quantity or number; "the inadequacy of
         unemployment benefits" [syn: insufficiency, inadequacy,
         deficiency] [ant: adequacy, sufficiency]
    2: a lack of competence; "pointed out the insufficiencies in my
       report"; "juvenile offenses often reflect an inadequacy in
       the parents" [syn: insufficiency, inadequacy]
    3: unsatisfactoriness by virtue of being inadequate [syn:
       inadequacy, inadequateness] [ant: adequacy,
       adequateness]

Moby Thesaurus II by Grady Ward, 1.0:

165 Moby Thesaurus words for "inadequacy":
   adulteration, arrear, arrearage, arrears, arrestment, asymmetry,
   baseness, blemish, bug, callowness, catch, contrariety, crack,
   decline, defalcation, default, defect, defectibility, defection,
   defectiveness, deficiency, deficit, delinquency, difference,
   disability, disablement, disequilibrium, disparity, disproportion,
   disqualification, dissatisfactoriness, drawback, erroneousness,
   failing, failure, fallibility, falling short, fault, faultiness,
   faute, fewness, flaw, foible, frailty, heterogeneity, hole,
   imbalance, imbecility, immaturity, impairment, imperfection,
   impurity, inability, inaccuracy, inadeptness, inadequateness,
   inadmissibility, inaptitude, inaptness, inattentiveness,
   incapability, incapacitation, incapacity, incompetence,
   incompetency, incompleteness, incompletion, indefensibility,
   indexterity, ineffectiveness, ineffectuality, ineffectualness,
   inefficacy, inefficiency, ineptitude, ineptness, inequality,
   inequity, inexactitude, inexactness, infancy, inferiority,
   infirmity, injustice, insufficiency, insufficientness,
   intolerability, irregularity, kink, lack, legal incapacity,
   little problem, littleness, maladjustment, maladroitness, meanness,
   mediocrity, minority, none to spare, nonfulfillment,
   nonsatisfaction, nonuniformity, objectionability, odds,
   overbalance, patchiness, pedestrianism, pettiness, problem, rift,
   rustiness, scantiness, scrappiness, shabbiness, short measure,
   short supply, short weight, shortage, shortcoming, shortfall,
   sketchiness, skill-lessness, slump, smallness, snag,
   something missing, subnormality, taint, thoughtlessness, too late,
   too little, triviality, unacceptability, unaptness, unbalance,
   uncleverness, undeftness, underage, underdevelopment,
   undesirability, undevelopment, undexterousness, undextrousness,
   unevenness, unfair discrimination, unfitness, unfittedness,
   unintelligence, unperfectedness, unproficiency, unqualification,
   unsatisfactoriness, unschooledness, unskillfulness, unsoundness,
   unsuitability, untenability, untrainedness, vulgarity,
   vulnerable place, want, wardship, weak link, weak point,
   weakness