The Collaborative International Dictionary of English v.0.48:

Shameless \Shame"less\, a. [AS. scamle['a]s.]
   1. Destitute of shame; wanting modesty; brazen-faced;
      insensible to disgrace. "Such shameless bards we have."
      --Pope.
      [1913 Webster]

            Shame enough to shame thee, wert thou not shameless.
                                                  --Shak.
      [1913 Webster]

   2. Indicating want of modesty, or sensibility to disgrace;
      indecent; as, a shameless picture or poem.
      [1913 Webster]

   Syn: Impudent; unblushing; audacious; immodest; indecent;
        indelicate.
        [1913 Webster] -- Shame"less*ly, adv. --
        Shame"less*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

shameless
    adj 1: feeling no shame; "a shameless imposter"; "an unblushing
           apologist for fascism" [syn: shameless, unblushing]

Moby Thesaurus II by Grady Ward, 1.0:

161 Moby Thesaurus words for "shameless":
   abandoned, aberrant, abnormal, abominable, amoral, arrant,
   atrocious, audacious, aweless, barefaced, blatant, bold,
   bold as brass, boldfaced, brash, brassy, brazen, brazenfaced,
   callous, calloused, case-hardened, cheeky, colorful,
   conscienceless, corrupt, corrupted, criminal, crooked, crude, dark,
   delinquent, deviant, devious, disgraceful, dishonest, dishonorable,
   dissolute, doubtful, dubious, evasive, evil, exhibitionistic,
   extravagant, felonious, fishy, flagrant, flaring, flaunting,
   forward, fraudulent, garish, gaudy, glaring, gorgeous, griefless,
   hard, hardened, hardhearted, hardly the thing, heartless,
   high-handed, ignominious, ill-got, ill-gotten, illegal, immodest,
   immoral, improper, impudent, inappropriate, incorrect, indecent,
   indecorous, indirect, indurated, infamous, insidious, inured, lewd,
   lost to shame, loud, lurid, meretricious, not done, not kosher,
   not the thing, notorious, obdurate, obtrusive, off-base, off-color,
   out-of-line, outrageous, overbold, presumptuous, profligate,
   questionable, regretless, remorseless, rotten, rude, sacrilegious,
   scandalous, screaming, seared, sensational, shady, shameful,
   shifty, shocking, sinful, sinister, slippery, sorrowless,
   spectacular, suspicious, swaggering, tawdry, terrible, tricky,
   unabashed, unashamed, unblushing, unconscienced, unconscientious,
   unconscionable, uncontrolled, underhand, underhanded, undue,
   unembarrassed, unethical, unfit, unfitting, ungrieving, unlawful,
   unmodest, unprincipled, unregretful, unregretting, unremorseful,
   unrepining, unreserved, unrighteous, unrueful, unsavory,
   unscrupulous, unseemly, unsorrowful, unsorrowing, unsorry,
   unstraightforward, unsuitable, vulgar, wanton, wicked, wild,
   without remorse, without shame, wrong, wrongful