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]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "incompetent":
   awkward, base, blank cartridge, bungling, castrato, clumsy,
   defective, deficient, disqualified, dud, dull tool, eunuch,
   failing, found wanting, gauche, gelding, greenhorn, ill-equipped,
   ill-fitted, ill-furnished, ill-provided, ill-qualified, imperfect,
   impotent, inadequate, incapable, incapable of, incomplete,
   ineffective, ineffectual, inefficient, ineligible, inept, inexpert,
   inferior, insufficient, invalid, lacking, little, maladjusted,
   maladroit, mean, mediocre, mediocrity, no conjuror, not comparable,
   not enough, not equal to, not in it, not up to, out of it, petty,
   shabby, small, too little, trivial, unable, unable to, unadapted,
   unadjusted, unarmed, unendowed, unequal to, unequipped, unfit,
   unfitted, ungifted, unprovided, unqualified, unsatisfactory,
   unsatisfying, unskilled, unskillful, unsufficing, unsuited,
   untalented, unworkmanlike, useless, wanting, weakling