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"