The Collaborative International Dictionary of English v.0.48:

Hilarious \Hi*la"ri*ous\, a. [L. hilaris, hilarus, Gr. ?; cf. ?
   gracious, kindly.]
   Mirthful; noisy; merry.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hilarious
    adj 1: marked by or causing boisterous merriment or convulsive
           laughter; "hilarious broad comedy"; "a screaming farce";
           "uproarious stories" [syn: hilarious, screaming(a),
           uproarious]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "hilarious":
   absurd, amusing, bizarre, buxom, cheerful, cheery, comical,
   convivial, droll, eccentric, entertaining, festal, festive,
   frivolous, funny, gala, gay, gladsome, gleeful, humorous,
   hysterical, incongruous, jocular, jocund, jolly, jovial, joyful,
   joyous, laughable, laughter-loving, ludicrous, merry, merrymaking,
   mirth-loving, mirthful, on the loose, priceless, quaint, quizzical,
   rejoicing, rich, ridiculous, risible, rollicking, screaming,
   uproarious, whimsical, witty