Source : Webster's Revised Unabridged Dictionary (1913)

Sudden \Sud"den\, n.
   An unexpected occurrence; a surprise.

   {All of a sudden}, {On a sudden}, {Of a sudden}, sooner than
      was expected; without the usual preparation; suddenly.

            How art thou lost! how on a sudden lost! --Milton.

            He withdrew his opposition all of a sudden.
                                                  --Thackeray.

Source : WordNet®

of a sudden
     adv : happening unexpectedly; "suddenly she felt a sharp pain in
           her side" [syn: {suddenly}, {all of a sudden}]