The Collaborative International Dictionary of English v.0.48:

Improper \Im*prop"er\, v. t.
   To appropriate; to limit. [Obs.]
   [1913 Webster]

         He would in like manner improper and inclose the
         sunbeams to comfort the rich and not the poor. --Jewel.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            Follow'd his enemy king, and did him service,
            Improper for a slave.                 --Shak.
      [1913 Webster]

            And to their proper operation still,
            Ascribe all Good; to their improper, Ill. --Pope.
      [1913 Webster]

   2. Not peculiar or appropriate to individuals; general;
      common. [Obs.]
      [1913 Webster]

            Not to be adorned with any art but such improper
            ones as nature is said to bestow, as singing and
            poetry.                               --J. Fletcher.
      [1913 Webster]

   3. Not according to facts; inaccurate; erroneous.
      [1913 Webster]

   Improper diphthong. See under Diphthong.

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

   Improper fraction. See under Fraction.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

257 Moby Thesaurus words for "improper":
   Doric, aberrant, abnormal, abominable, amiss, arrant, atrocious,
   bad, barbaric, barbarous, base, black, blamable, blameworthy,
   blooper, boner, boo-boo, break, cacophonous, careless, chintzy,
   clumsy, coarse, corrupt, corruption, criminal, crude, damnable,
   dark, delinquent, deviant, disgraceful, disorderly, disruptive,
   doggerel, dysphemistic, erroneous, evil, excessive, execrable,
   false, faulty, faux pas, flagitious, flagrant, foul, fresh, futile,
   gaffe, gauche, graceless, gross, hardly the thing, harsh, heinous,
   ignominious, ill, ill-adapted, ill-advised, ill-assorted,
   ill-chosen, ill-considered, ill-fitted, ill-matched, ill-seasoned,
   ill-sorted, ill-suited, ill-timed, illegal, immodest, immoral,
   impertinent, impolite, impolitic, imprecise, impropriety, impure,
   in bad taste, inaccurate, inadmissibility, inadmissible,
   inadvisable, inapplicable, inapposite, inappropriate, inapt,
   inauspicious, incompatible, inconcinnate, inconcinnous,
   incongruous, inconvenient, incorrect, incorrectness, indecent,
   indecorous, indecorousness, indecorum, indelicate, indiscreet,
   inelegance, inelegant, inept, inexact, inexpedient, infamous,
   infelicitous, inferior, informal, iniquitous, inopportune,
   intempestive, intrusive, invalid, irregular, irrelevant, knavish,
   lascivious, late, lewd, loose, low, mal a propos, maladjusted,
   malapropos, malevolent, malodorous, misbehaving, misjoined,
   mismatched, mismated, misplaced, mistaken, mistimed, monstrous,
   naughty, nefarious, not done, not respectable, not the thing,
   objectionableness, obscene, off base, off color, off-base,
   off-color, offensive, out of character, out of joint,
   out of keeping, out of line, out of phase, out of place,
   out of proportion, out of season, out of time, out of tune,
   out-of-line, outlandish, peccant, premature, rank, reprehensible,
   reprobate, risque, rough, rowdy, rowdyish, rude, ruffianly,
   sacrilegious, sassy, scandalous, shameful, shameless, sinful,
   sinister, slipshod, slovenly, solecism, solecistic, suggestive,
   tactless, tasteless, terrible, too late, too soon,
   unacceptableness, unadapted, unapt, unbecoming, unbecomingness,
   unbefitting, unbehaving, unbeseeming, uncalled-for, unceremonious,
   uncomely, unconventional, uncourtly, uncouth, undecorous,
   undesirable, undignified, undue, uneuphonious, unfavorable,
   unfelicitous, unfit, unfitted, unfitting, unforgivable,
   unfortunate, ungenteel, ungodly, ungraceful, ungrammatic, unhandy,
   unhappy, unhealthy, unkind, unlawful, unlucky, unmeet, unmeetness,
   unpardonable, unpleasant, unpolished, unprofitable, unpropitious,
   unqualified, unready, unrefined, unrighteous, unripe, unseasonable,
   unseemliness, unseemly, unskillful, unspeakable, unsuitable,
   unsuited, untasteful, untimely, untoward, untrue, unwise, unworthy,
   vernacularism, vicious, vile, villainous, vulgar, vulgarism,
   wicked, wrong, wrongful