Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Sudden fear troubleth thee.           --Job xxii.
                                                  10.

   2. Hastly prepared or employed; quick; rapid.

            Never was such a sudden scholar made. --Shak.

            The apples of Asphaltis, appearing goodly to the
            sudden eye.                           --Milton.

   3. Hasty; violent; rash; precipitate. [Obs.] --Shak.

   Syn: Unexpected; unusual; abrupt; unlooked-for. --
        {Sud"den*ly}, adv. -- {Sud"den*ness}, n.

Source : WordNet®

suddenly
     adv 1: happening unexpectedly; "suddenly she felt a sharp pain in
            her side" [syn: {all of a sudden}, {of a sudden}]
     2: quickly and without warning; "he stopped suddenly" [syn: {abruptly},
         {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}]