The Collaborative International Dictionary of English v.0.48:

Unbecoming \Un`be*com"ing\, a. [Pref. un- not + becoming.]
   Not becoming; unsuitable; unfit; indecorous; improper.
   [1913 Webster]

         My grief lets unbecoming speeches fall.  --Dryden.
   [1913 Webster] -- Un`be*com"ing*ly, adv. --
   Un`be*com"ing*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unbecoming
    adj 1: not in keeping with accepted standards of what is right
           or proper in polite society; "was buried with indecent
           haste"; "indecorous behavior"; "language unbecoming to a
           lady"; "unseemly to use profanity"; "moved to curb their
           untoward ribaldry" [syn: indecent, indecorous,
           unbecoming, uncomely, unseemly, untoward]

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "unbecoming":
   awkward, beneath one, cheap, chintzy, clumsy, debasing, degrading,
   demeaning, deplorable, disgraceful, gauche, gutter, humiliating,
   humiliative, ill-adapted, ill-assorted, ill-chosen, ill-fitted,
   ill-matched, ill-sorted, ill-suited, ill-timed, improper,
   in bad taste, inadmissible, inapplicable, inapposite,
   inappropriate, inapt, indecent, indecorous, indelicate, indiscreet,
   inelegant, inept, infelicitous, infra dig, infra indignitatem,
   irrelevant, mal a propos, maladjusted, maladroit, malapropos,
   malodorous, misjoined, mismatched, mismated, misplaced, offensive,
   opprobrious, out of character, out of joint, out of keeping,
   out of line, out of place, out of proportion, out of season,
   out of time, out of tune, outrageous, pitiful, rough, sad,
   scandalous, shameful, shocking, sorry, tasteless, too bad,
   unadapted, unapt, unbefitting, unbeseeming, undecorous,
   undignified, undue, unfit, unfitted, unfitting, ungenteel,
   ungentlemanly, ungodly, unladylike, unqualified, unseasonable,
   unseemly, unsuitable, unsuited, untasteful, untimely, untoward,
   unworthy of one, vulgar