The Collaborative International Dictionary of English v.0.48:

Incompetent \In*com"pe*tent\, a. [L. incompetens: cf. F.
   incomp['e]tent. See In- not, and Competent.]
   [1913 Webster]
   1. Not competent; wanting in adequate strength, power,
      capacity, means, qualifications, or the like; incapable;
      unable; inadequate; unfit.
      [1913 Webster]

            Incompetent to perform the duties of the place.
                                                  --Macaulay.
      [1913 Webster]

   2. (Law) Wanting the legal or constitutional qualifications;
      inadmissible; as, a person professedly wanting in
      religious belief is an incompetent witness in a court of
      law or equity; incompetent evidence; a mentally defective
      person is incompetent to care for himself and requires a
      legal guardian.
      [1913 Webster +PJC]

            Richard III. had a resolution, out of hatred to his
            brethren, to disable their issues, upon false and
            incompetent pretexts, the one of attainder, the
            other of illegitimation.              --Bacon.
      [1913 Webster]

   3. Not lying within one's competency, capacity, or authorized
      power; not permissible.

   Syn: Incapable; unable; inadequate; insufficient;
        inefficient; disqualified; unfit; improper.

   Usage: Incompetent, Incapable. Incompetent is a relative
          term, denoting a lack of the requisite qualifications
          for performing a given act, service, etc.; incapable
          is absolute in its meaning, denoting lack of power,
          either natural or moral. We speak of a man as
          incompetent to a certain task, of an incompetent
          judge, etc. We say of an idiot that he is incapable of
          learning to read; and of a man distinguished for his
          honor, that he is incapable of a mean action.
          [1913 Webster]

WordNet (r) 3.0 (2006):

incompetent
    adj 1: legally not qualified or sufficient; "a wife is usually
           considered unqualified to testify against her husband";
           "incompetent witnesses" [syn: incompetent,
           unqualified] [ant: competent]
    2: not qualified or suited for a purpose; "an incompetent secret
       service"; "the filming was hopeless incompetent" [ant:
       competent]
    3: showing lack of skill or aptitude; "a bungling workman"; "did
       a clumsy job"; "his fumbling attempt to put up a shelf" [syn:
       bungling, clumsy, fumbling, incompetent]
    4: not doing a good job; "incompetent at chess" [syn:
       incompetent, unskilled]
    5: not meeting requirements; "unequal to the demands put upon
       him" [syn: incapable, incompetent, unequal to(p)]
    n 1: someone who is not competent to take effective action [syn:
         incompetent, incompetent person]