Of a sudden

from WordNet (r) 3.0 (2006)

of a sudden
    adv 1: happening unexpectedly; "suddenly she felt a sharp pain
           in her side" [syn: {suddenly}, {all of a sudden}, {of a
           sudden}]

from 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]