The Collaborative International Dictionary of English v.0.48:

Unawares \Un`a*wares"\, adv.
   Without design or preparation; suddenly; without
   premeditation, unexpectedly. "Mercies lighting unawares."
   --J. H. Newman.
   [1913 Webster]

         Lest unawares we lose
         This our high place, our sanctuary, our hill. --Milton.
   [1913 Webster]

   At unaware, or At unawares, unexpectedly; by surprise.
      [1913 Webster]

            He breaks at unawares upon our walks. --Dryden.
      [1913 Webster]

            So we met
            In this old sleepy town at unaware.   --R. Browning.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unawares
    adv 1: without forethought or plan; inadvertently; "came upon
           the diamond unawares"
    2: suddenly and unexpectedly; "rain caught them unawares";
       "sorrow comes to all, and to the young it comes with bittered
       agony because it takes them unawares"- A.Lincoln
    3: at a disadvantage; "I was caught short" [syn: short,
       unawares]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "unawares":
   aback, abruptly, accidentally, bang, by accident, by surprise,
   dash, guiltlessly, hastily, ignorantly, impetuously, implausibly,
   improbably, impulsively, inadvertently, innocently, insensibly,
   like a flash, like a thunderbolt, mistakenly, of a sudden,
   off base, on short notice, plop, plump, plunk, pop, precipitantly,
   precipitately, precipitously, sharp, short, slap, smack,
   startlingly, sudden, suddenly, surprisingly, unanticipatedly,
   unaware, unconsciously, unexpectedly, unforeseeably, unguiltily,
   unintentionally, unknowingly, unmindfully, unpredictably,
   unprepared, unready, unsuspectingly, unwittingly, without notice,
   without warning, witlessly