The Collaborative International Dictionary of English v.0.48:

Sudden \Sud"den\, n.
   An unexpected occurrence; a surprise.
   [1913 Webster]

   All of a sudden, On a sudden, Of a sudden, sooner than
      was expected; without the usual preparation; suddenly.
      [1913 Webster]

            How art thou lost! how on a sudden lost! --Milton.
      [1913 Webster]

            He withdrew his opposition all of a sudden.
                                                  --Thackeray.
      [1913 Webster]

WordNet (r) 3.0 (2006):

all of a sudden
    adv 1: without warning; "all at once, he started shouting" [syn:
           all of a sudden, all at once]
    2: happening unexpectedly; "suddenly she felt a sharp pain in
       her side" [syn: suddenly, all of a sudden, of a sudden]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "all of a sudden":
   abruptly, bang, dash, hastily, impetuously, impulsively,
   like a flash, like a thunderbolt, of a sudden, on short notice,
   plop, plump, plunk, pop, precipitantly, precipitately,
   precipitously, sharp, short, slap, smack, startlingly, sudden,
   suddenly, surprisingly, unawares, unexpectedly, without notice,
   without warning