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):

recklessness
    n 1: the trait of giving little thought to danger [syn:
         recklessness, foolhardiness, rashness]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "recklessness":
   abandon, accidia, acedia, apathy, ataraxia, ataraxy, callowness,
   careless abandon, carelessness, casualness, childishness,
   cursoriness, desperateness, detachment, devil-may-careness,
   disinterest, dispassion, disregard, disregardfulness, easiness,
   easygoingness, forgetfulness, furiousness, haste, hastiness,
   heedlessness, hotheadedness, hurriedness, immaturity, impatience,
   impetuosity, impetuousness, imprudence, impulsiveness,
   inadvisability, inattention, inconsiderateness, inconsideration,
   incuriosity, indifference, indiscreetness, indiscretion,
   indiscrimination, inexcitability, inexpedience, injudiciousness,
   insouciance, irrationality, lack of affect, lack of foresight,
   lackadaisicalness, laziness, listlessness, mindlessness,
   negligence, nonchalance, oblivion, offhandedness, overeagerness,
   overenthusiasm, overhastiness, overzealousness, perfunctoriness,
   pococurantism, pompousness, precipitance, precipitancy,
   precipitateness, precipitation, precipitousness, puerility,
   quickness, rashness, reasonlessness, regardlessness, senselessness,
   sloth, sprezzatura, stuffiness, suddenness, tactlessness,
   thoughtlessness, unanxiousness, unconcern, unheedfulness,
   unintelligence, unmindfulness, unpreparedness, unreadiness,
   unreason, unreasonableness, unsensibleness, unsolicitousness,
   unsolicitude, unsoundness, unthinkingness, unthoughtfulness,
   unwisdom, unwiseness, wantonness, wildness, witlessness