The Collaborative International Dictionary of English v.0.48:

hysterical \hys*ter"ic*al\ a.
   Extremely funny. [Colloq.]
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Hysteric \Hys*ter"ic\, Hysterical \Hys*ter"ic*al\, a. [L.
   hystericus, Gr. ?, fr. "yste`ra the womb; perh. akin to ?
   latter, later, and E. utter, out.]
   1. Of or pertaining to hysteria.
      [1913 Webster]

            With no hysteric weakness or feverish excitement,
            they preserved their peace and patience. --Bancroft.
      [1913 Webster]

   2. In a state of hysteria; affected, or troubled, with
      hysterics; uncontrollably emotional; convulsive, fitful.
      [1913 Webster]

   3. In a state of panic or behaving in a wild irrational
      manner, due to fear or emotional trauma.
      [PJC]

   4. Resembling hysteria; as, hysterical laughter.
      [PJC]

WordNet (r) 3.0 (2006):

hysterical
    adj 1: characterized by or arising from psychoneurotic hysteria;
           "during hysterical conditions various functions of the
           human body are disordered"- Morris Fishbein; "hysterical
           amnesia" [syn: hysteric, hysterical]
    2: marked by excessive or uncontrollable emotion; "hysterical
       laughter"; "a mob of hysterical vigilantes"

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "hysterical":
   abandoned, amok, at fever pitch, bellowing, berserk, carried away,
   comical, crazed, delirious, demoniac, disordered, distracted,
   disturbed, ecstatic, emotionalistic, emotive, enraptured,
   fanatical, farcical, febrile, feral, ferocious, feverish, fierce,
   frantic, frenetic, frenzied, fulminating, funny, furious, haggard,
   hammy, hectic, hilarious, histrionic, hog-wild, howling,
   hypochondriac, hysteric, in a transport, in hysterics, infatuated,
   insane, intoxicated, irrational, mad, madding, maniac,
   melodramatic, neurasthenic, neurotic, nonrational, orgasmic,
   orgiastic, overanxious, overdesirous, overeager, overemotional,
   overenthusiastic, overzealous, perfervid, phobic, possessed,
   psychasthenic, psychoneurotic, rabid, raging, ramping, ranting,
   raving, ravished, roaring, running mad, sensational,
   sensationalistic, storming, theatrical, transported, ultrazealous,
   uncontrollable, uncontrolled, unreasoning, unrestrained,
   uproarious, violent, wild, wild-eyed, wild-looking