The Collaborative International Dictionary of English v.0.48:

Inefficiency \In`ef*fi"cien*cy\, n.
   The quality of being inefficient; lack of power or energy
   sufficient for the desired effect; inefficacy; incapacity;
   as, he was discharged from his position for inefficiency.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inefficiency
    n 1: unskillfulness resulting from a lack of efficiency [ant:
         efficiency]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "inefficiency":
   disability, disablement, disqualification, imbecility, inability,
   inadeptness, inadequacy, inaptitude, inaptness, inattentiveness,
   incapability, incapacitation, incapacity, incompetence,
   incompetency, indexterity, ineffectiveness, ineffectuality,
   ineptitude, ineptness, infancy, inferiority, insufficiency,
   legal incapacity, maladjustment, maladroitness, mediocrity,
   minority, pedestrianism, rustiness, skill-lessness,
   thoughtlessness, unaptness, uncleverness, undeftness,
   undexterousness, undextrousness, unfitness, unfittedness,
   unintelligence, unproficiency, unschooledness, unskillfulness,
   untrainedness, wardship