Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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