Source : Webster's Revised Unabridged Dictionary (1913)

Improper \Im*prop"er\, a. [F. impropre, L. improprius; pref. im-
   not + proprius proper. See {Proper}.]
   1. Not proper; not suitable; not fitted to the circumstances,
      design, or end; unfit; not becoming; incongruous;
      inappropriate; indecent; as, an improper medicine;
      improper thought, behavior, language, dress.

            Follow'd his enemy king, and did him service,
            Improper for a slave.                 --Shak.

            And to their proper operation still, Ascribe all
            Good; to their improper, Ill.         --Pope.

   2. Not peculiar or appropriate to individuals; general;
      common. [Obs.]

            Not to be adorned with any art but such improper
            ones as nature is said to bestow, as singing and
            poetry.                               --J. Fletcher.

   3. Not according to facts; inaccurate; erroneous.

   {Improper diphthong}. See under {Diphthong}.

   {Improper feud}, an original feud, not earned by military
      service. --Mozley & W.

   {Improper fraction}. See under {Fraction}.

Improper \Im*prop"er\, v. t.
   To appropriate; to limit. [Obs.]

         He would in like manner improper and inclose the
         sunbeams to comfort the rich and not the poor. --Jewel.

Source : WordNet®

improper
     adj 1: not suitable or right or appropriate; "slightly improper to
            dine alone with a married man"; "improper medication"
            [ant: {proper}]
     2: not conforming to legality, moral law, or social convention;
        "an unconventional marriage"; "improper banking practices"
        [syn: {unconventional}, {unlawful}]
     3: not appropriate for a purpose or occasion; "unsuitable
        attire for the office"; "said all the wrong things" [syn:
        {unsuitable}, {wrong}]