The Collaborative International Dictionary of English v.0.48:

Histrionic \His`tri*on"ic\, Histrionical \His`tri*on"ic*al\, a.
   [L. histrionicus: cf. F. histrionique. See Histrion.]
   1. Of or relating to the stage or a stageplayer; befitting a
      theatre; theatrical.
      [1913 Webster]

   2. Affectedly dramatic; insincerely emotional; -- sometimes
      used in a bad sense. -- His`tri*on"ic*al*ly, adv.
      [1913 Webster]

            Tainted with false and histrionic feeling. --De
                                                  Quincey.
      [1913 Webster]

WordNet (r) 3.0 (2006):

histrionic
    adj 1: characteristic of acting or a stage performance; often
           affected; "histrionic gestures"; "an attitude of
           melodramatic despair"; "a theatrical pose" [syn:
           histrionic, melodramatic]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "histrionic":
   actor-proof, affected, all-star, artificial, ballet, balletic,
   cinematic, cinematographic, dramatic, dramatical, dramaturgic,
   emotionalistic, emotive, film, filmic, ham, hammy, hysteric,
   insincere, la-di-da, legitimate, maniere, mannered, melodramatic,
   milked, monodramatic, movie, nonrational, operatic, overacted,
   overdone, overemotional, overplayed, pretentious, scenic,
   sensational, sensationalistic, spectacular, stagelike, stageworthy,
   stagy, starstruck, stellar, theaterlike, theatrical, thespian,
   thrown away, underacted, underplayed, unnatural, unreasoning,
   vaudevillian