The Collaborative International Dictionary of English v.0.48:

Inappropriate \In`ap*pro"pri*ate\, a.
   Not instrument (to); not appropriate; unbecoming; unsuitable;
   not specially fitted; -- followed by to or for. --
   In`ap*pro"pri*ate*ly, adv. -- In`ap*pro"pri*ate*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inappropriate
    adj 1: not suitable for a particular occasion etc; "noise seems
           inappropriate at a time of sadness"; "inappropriate shoes
           for a walk on the beach"; "put inappropriate pressure on
           them" [ant: appropriate]
    2: not in keeping with what is correct or proper; "completely
       inappropriate behavior" [syn: inappropriate,
       incompatible, out or keeping(p), unfitting]

Moby Thesaurus II by Grady Ward, 1.0:

150 Moby Thesaurus words for "inappropriate":
   aberrant, abnormal, abominable, adrift, atrocious, bad,
   beside the mark, beside the point, beside the question, chintzy,
   criminal, delinquent, deviant, discordant, disgraceful, evil,
   excessive, extraneous, extrinsic, futile, hardly the thing,
   ignominious, ill-adapted, ill-advised, ill-assorted, ill-chosen,
   ill-considered, ill-fitted, ill-matched, ill-seasoned, ill-sorted,
   ill-suited, ill-timed, illegal, immaterial, impertinent, impolitic,
   improper, in bad taste, inadmissible, inadvisable, inapplicable,
   inapposite, inapt, inauspicious, incidental, incompatible,
   incongruous, inconsequent, inconsonant, inconvenient, incorrect,
   indecent, indecorous, indelicate, indiscreet, inelegant, inept,
   inexpedient, infamous, infelicitous, inharmonious, inopportune,
   intempestive, intrusive, irrelative, irrelevant, late,
   mal a propos, maladjusted, malapropos, misjoined, mismatched,
   mismated, misplaced, mistimed, nihil ad rem, nonessential,
   not at issue, not done, not the thing, off base, off the subject,
   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, out-of-the-way, parenthetical, premature,
   sacrilegious, scandalous, shameful, shameless, sinful, tasteless,
   terrible, too late, too soon, unadapted, unapt, unbecoming,
   unbefitting, unbeseeming, uncongenial, undesirable, undignified,
   undue, unessential, unfavorable, unfit, unfitted, unfitting,
   unfortunate, ungenteel, unhandy, unhappy, unharmonious, unlawful,
   unlucky, unmeet, unprofitable, unpropitious, unqualified, unready,
   unrighteous, unripe, unseasonable, unseemly, unstable, unsuitable,
   unsuited, untasteful, untimely, untoward, unwise, vulgar, wicked,
   wrong, wrongful