The Collaborative International Dictionary of English v.0.48:

Regardless \Re*gard"less\, a.
   1. Having no regard; heedless; careless; as, regardless of
      life, consequences, dignity.
      [1913 Webster]

            Regardless of the bliss wherein he sat. --Milton.
      [1913 Webster]

   2. Not regarded; slighted. [R.] --Spectator.
      [1913 Webster]

   Syn: Heedless; negligent; careless; indifferent; unconcerned;
        inattentive; unobservant; neglectful.
        [1913 Webster] -- Re*gard"less*ly, adv. --
        Re*gard"less*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

regardless
    adv 1: in spite of everything; without regard to drawbacks; "he
           carried on regardless of the difficulties" [syn:
           regardless, irrespective, disregardless, no
           matter, disregarding]
    adj 1: (usually followed by `of') without due thought or
           consideration; "careless of the consequences"; "crushing
           the blooms with regardless tread" [syn: careless(p),
           regardless]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "regardless":
   airy, aloof, anyhow, anyway, anywise, apathetic, at all,
   at all costs, at all events, at any cost, at any rate, ataractic,
   behindhand, blase, bored, by any means, careless, casual, cursory,
   degage, delinquent, derelict, despite, detached, devil-may-care,
   disinterested, dispassionate, disregardant, disregardful, distant,
   distracted, distrait, easygoing, even with, flippant, forgetful,
   free and easy, heedless, however, impassive, in any case,
   in any event, in any way, in despite of, in spite of, inadvertent,
   inattentive, inconsiderate, incurious, indifferent, inexcitable,
   inobservant, insouciant, irregardless, irrespective of,
   lackadaisical, lax, lazy, listless, mindless, neglectful,
   negligent, nevertheless, no matter what, nohow, nonchalant,
   nonetheless, notwithstanding, oblivious, offhand, perfunctory,
   phlegmatic, pococurante, reckless, regardless of,
   regardless of cost, remiss, respectless, slack, spite of, stolid,
   tactless, thoughtless, turned-off, unanxious, unconcerned,
   undiplomatic, undiscriminating, unheedful, unheeding, uninquiring,
   uninterested, uninvolved, unmarking, unmindful, unnoticing,
   unnoting, unobservant, unobserving, unprepared, unready,
   unremarking, unsolicitous, untactful, unthinking, with,
   withdrawn