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):

reckless
    adj 1: marked by defiant disregard for danger or consequences;
           "foolhardy enough to try to seize the gun from the
           hijacker"; "became the fiercest and most reckless of
           partisans"-Macaulay; "a reckless driver"; "a rash attempt
           to climb Mount Everest" [syn: foolhardy, heady,
           rash, reckless]
    2: characterized by careless unconcern; "the heedless generosity
       and the spasmodic extravagance of persons used to large
       fortunes"- Edith Wharton; "reckless squandering of public
       funds" [syn: heedless, reckless]

Moby Thesaurus II by Grady Ward, 1.0:

157 Moby Thesaurus words for "reckless":
   accident-prone, adventuresome, agile, airy, apathetic, ataractic,
   audacious, blase, brash, breakneck, carefree, careless, casual,
   cursory, dangerous, daredevil, daring, dashing, degage, desperate,
   devil-may-care, disinterested, dispassionate, disregardant,
   disregardful, double-quick, eagle-winged, easygoing, expeditious,
   express, fast, feckless, fleet, flippant, flying, foolhardy,
   foolish, forgetful, free and easy, furious, galloping,
   hair-trigger, harum-scarum, hasty, headlong, heedless, hopeless,
   hotheaded, hurried, hustling, ill-advised, ill-considered,
   ill-contrived, ill-devised, ill-gauged, ill-judged, impatient,
   impetuous, impolitic, imprudent, impulsive, inadvisable,
   inattentive, incautious, inconsiderate, incurious, indifferent,
   indiscreet, inexcitable, inexpedient, injudicious, insensate,
   insouciant, irrational, irresponsible, lackadaisical, lazy,
   light of heel, light-footed, listless, lively, mad, madcap,
   mercurial, mindless, misadvised, misguided, myopic, negligent,
   nimble, nimble-footed, nonchalant, oblivious, offhand, overeager,
   overenthusiastic, overhasty, overzealous, perfunctory, pococurante,
   precipitant, precipitate, precipitous, prompt, quick,
   quick as lightning, quick as thought, rapid, rash, reasonless,
   regardless, respectless, running, senseless, shortsighted,
   slap-bang, slapdash, snappy, spanking, speedy, sudden, swift,
   tactless, temerarious, thoughtless, turned-off, unadvised,
   unanxious, unconcerned, unconsidered, undiplomatic, undiscerning,
   undiscriminating, unforeseeing, unheedful, unheeding, uninterested,
   unmindful, unprepared, unready, unreasonable, unreflecting,
   unreflective, unseeing, unsensible, unsolicitous, unsound,
   untactful, unthinking, unthoughtful, unwise, venturesome,
   venturous, wanton, wild, winged, witless