The Collaborative International Dictionary of English v.0.48:

Sudden \Sud"den\, a. [OE. sodian, sodein, OF. sodain, sudain, F.
   soudain, L. subitaneus, fr. subitus sudden, that has come
   unexpectedly, p. p. of subire to come on, to steal upon; sub
   under, secretly + ire to go. See Issue, and cf.
   Subitaneous.]
   1. Happening without previous notice or with very brief
      notice; coming unexpectedly, or without the common
      preparation; immediate; instant; speedy. "O sudden wo!"
      --Chaucer. "For fear of sudden death." --Shak.
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            Sudden fear troubleth thee.           --Job xxii.
                                                  10.
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   2. Hastly prepared or employed; quick; rapid.
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            Never was such a sudden scholar made. --Shak.
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            The apples of Asphaltis, appearing goodly to the
            sudden eye.                           --Milton.
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   3. Hasty; violent; rash; precipitate. [Obs.] --Shak.
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   Syn: Unexpected; unusual; abrupt; unlooked-for.
        [1913 Webster] -- Sud"den*ly, adv. -- Sud"den*ness,
        n.
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WordNet (r) 3.0 (2006):

suddenly
    adv 1: happening unexpectedly; "suddenly she felt a sharp pain
           in her side" [syn: suddenly, all of a sudden, of a
           sudden]
    2: quickly and without warning; "he stopped suddenly" [syn:
       abruptly, suddenly, short, dead]
    3: on impulse; without premeditation; "he decided to go to
       Chicago on the spur of the moment"; "he made up his mind
       suddenly" [syn: on the spur of the moment, suddenly]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "suddenly":
   a corps perdu, aback, abruptly, all at once, bang, by surprise,
   dash, feverishly, fleetingly, forthwith, hastily, head over heels,
   headfirst, headforemost, headlong, heels over head, hurriedly,
   impetuously, implausibly, improbably, impulsively, in a trice,
   instantaneously, instantly, like a flash, like a thunderbolt,
   momentarily, of a sudden, on short notice, plop, plump, plunk, pop,
   precipitantly, precipitately, precipitously, quickly, rapidly,
   rashly, recklessly, sharp, short, slap, slap-bang, smack, speedily,
   startlingly, sudden, surprisingly, swiftly, unanticipatedly,
   unaware, unawares, unexpectedly, unforeseeably, unpredictably,
   without notice, without warning