The Collaborative International Dictionary of English v.0.48:

Inaptitude \In*apt"i*tude\, n. [In- + aptitude: cf. F.
   inaptitude. Cf. Ineptitude.]
   Lack of aptitude.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inaptitude
    n 1: a lack of aptitude [ant: aptitude]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "inaptitude":
   abnormality, anomaly, futility, impoliticalness, impoliticness,
   impropriety, inability, inadeptness, inadequacy, inadmissibility,
   inadvisability, inapplicability, inappositeness, inappropriateness,
   inaptness, inattentiveness, incapability, incapacity, incompetence,
   incompetency, incongruity, inconvenience, inconveniency,
   indexterity, ineffectiveness, ineffectuality, inefficiency,
   ineptitude, ineptness, inexpedience, inexpediency, infelicity,
   inopportuneness, irrelevance, irrelevancy, maladjustment,
   maladroitness, mediocrity, mesalliance, misalliance, misjoinder,
   misjoining, mismatch, pedestrianism, rustiness, skill-lessness,
   thoughtlessness, unaptness, uncleverness, uncongeniality,
   undeftness, undesirability, undexterousness, undextrousness,
   unfitness, unfittedness, unfittingness, unfortunateness,
   unintelligence, unmeetness, unproficiency, unprofitability,
   unprofitableness, unschooledness, unseasonableness, unseemliness,
   unskillfulness, unsuitability, untimeliness, untrainedness,
   unwiseness, uselessness, worthlessness, wrongness