Source : Webster's Revised Unabridged Dictionary (1913)

Unconscionable \Un*con"scion*a*ble\, a.
   1. Not conscionable; not conforming to reason; unreasonable;
      exceeding the limits of any reasonable claim or
      expectation; inordinate; as, an unconscionable person or
      demand; unconscionable size.

            Which use of reason, most reasonless and
            unconscionable, is the utmost that any tyrant ever
            pretended.                            --Milton.

            His giantship is gone somewhat crestfallen, Stalking
            with less unconscionable strides.     --Milton.

   2. Not guided by, or conformed to, conscience. [Obs.]

            Ungenerous as well as unconscionable practices.
                                                  --South.
      -- {Un*con"scion*a*ble*ness}, n. -- {Un*con"scion*a*bly},
      adv.

Source : WordNet®

unconscionable
     adj 1: lacking a conscience; "a conscienceless villain"; "brash,
            unprincipled, and conscienceless"; "an unconscionable
            liar" [syn: {conscienceless}]
     2: greatly exceeding bounds of reason or moderation;
        "exorbitant rent"; "extortionate prices"; "spends an
        outrageous amount on entertainment"; "usorious interest
        rate"; "unconscionable spending" [syn: {exorbitant}, {extortionate},
         {outrageous}, {steep}, {usurious}]