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]