The Collaborative International Dictionary of English v.0.48:

Reckless \Reck"less\, a. [AS. reccele['a]s, r[=e]cele['a]s.]
   1. Inattentive to duty; careless; neglectful; indifferent.
      --Chaucer.
      [1913 Webster]

   2. Rashly negligent; utterly careless or heedless.
      [1913 Webster]

            It made the king as reckless as them diligent. --Sir
                                                  P. Sidney.
      [1913 Webster]

   Syn: Heedless; careless; mindless; thoughtless; negligent;
        indifferent; regardless; unconcerned; inattentive;
        remiss; rash.
        [1913 Webster] -- Reck"less*ly, adv. --
        Reck"less*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

recklessly
    adv 1: in a reckless manner

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "recklessly":
   a corps perdu, airily, any old way, any which way, anyhow,
   apathetically, bunglingly, carelessly, casually, clumsily,
   cursorily, desperately, disinterestedly, dispassionately,
   disregardfully, forgetfully, furiously, haphazardly,
   happen what may, hastily, head over heels, headfirst, headforemost,
   headlong, heedlessly, heels over head, helter-skelter,
   hit and miss, hit or miss, holus-bolus, hotheadedly, hurriedly,
   hurry-scurry, impetuously, impulsively, inattentively,
   inconsiderately, insouciantly, lackadaisically, like crazy,
   like mad, listlessly, madly, messily, mindlessly, negligently,
   nonchalantly, offhand, offhandedly, once over lightly, overeagerly,
   overenthusiastically, overzealously, perfunctorily, precipitantly,
   precipitately, precipitously, promiscuously, quickly,
   ramble-scramble, rashly, regardlessly, slam-bang, slap-bang,
   slapdash, sloppily, suddenly, tactlessly, thoughtlessly,
   unconcernedly, unguardedly, unheedfully, unheedingly,
   uninterestedly, unmindfully, unsolicitously, unthinkingly,
   unvigilantly, unwarily, wantonly, wildly