Source : Webster's Revised Unabridged Dictionary (1913)

Conscienceless \Con"science*less\, a.
   Without conscience; indifferent to conscience; unscrupulous.

         Conscienceless and wicked patrons.       --Hookre.

Source : WordNet®

conscienceless
     adj : lacking a conscience; "a conscienceless villain"; "brash,
           unprincipled, and conscienceless"; "an unconscionable
           liar" [syn: {unconscionable}]