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]