The Collaborative International Dictionary of English v.0.48:

Disregardful \Dis`re*gard"ful\, a.
   Neglect; negligent; heedless; regardless.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "disregardful":
   airy, apathetic, arrogant, ataractic, audacious, behindhand, blase,
   bold, brash, brassy, brazen, bumptious, careless, casual,
   challenging, cheeky, cocky, contemptuous, cursory, daring, defiant,
   defying, degage, derisive, devil-may-care, disdainful,
   disinterested, dispassionate, disregardant, distracted, distrait,
   easygoing, flippant, forgetful, free and easy, greatly daring,
   heedless, impertinent, impudent, inadvertent, inattentive,
   inconsiderate, inconstant, incurious, indifferent, inexcitable,
   inobservant, insolent, insouciant, lackadaisical, lazy, listless,
   mindless, negligent, nonadherent, nonchalant, noncompliant,
   nonconforming, nonobservant, oblivious, offhand, perfunctory, pert,
   pococurante, reckless, regardless, regardless of consequences,
   remiss, respectless, saucy, slack, tactless, thoughtless,
   turned-off, unaccommodating, unanxious, uncomplaisant, uncompliant,
   unconcerned, unconforming, uncooperative, undiplomatic,
   undiscriminating, unfaithful, unheedful, unheeding, unhelpful,
   uninterested, unloyal, unmarking, unmindful, unnoticing, unnoting,
   unobliging, unobservant, unobserving, unprepared, unready,
   unremarking, unsolicitous, untactful, unthinking, unthoughtful,
   untrue