The Collaborative International Dictionary of English v.0.48:

Unconscionable \Un*con"scion*a*ble\
   ([u^]n*k[o^]n"sh[u^]n*[.a]*b'l), a.
   1. Not conscionable; exceeding the limits of any reasonable
      claim or expectation; not conforming to reason;
      unreasonable; inordinate; extravagant; as, an
      unconscionable person or demand; unconscionable size.
      [1913 Webster]

            Which use of reason, most reasonless and
            unconscionable, is the utmost that any tyrant ever
            pretended.                            --Milton.
      [1913 Webster]

            His giantship is gone somewhat crestfallen,
            Stalking with less unconscionable strides. --Milton.
      [1913 Webster]

   2. Not guided by, or conformed to, conscience; that cannot be
      done in good conscience; as, unconscionable profits.
      [1913 Webster +PJC]

            Ungenerous as well as unconscionable practices.
                                                  --South.
      [1913 Webster] -- Un*con"scion*a*ble*ness, n. --
      Un*con"scion*a*bly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

175 Moby Thesaurus words for "unconscionable":
   a bit much, abandoned, absolute, acute, amoral, arrant, barbarous,
   boundless, classical, complete, conscienceless, consummate,
   corrupt, corrupted, crass, criminal, crooked, cutthroat, cutting,
   dark, decided, definitive, devious, dishonest, dishonorable, dizzy,
   doubtful, downright, drastic, dubious, egregious, enormous,
   evasive, evil, exacting, exaggerated, excessive, exorbitant,
   extortionate, extravagant, extreme, fabulous, fancy, felonious,
   fierce, fishy, flagrant, fraudulent, furious, gigantic, glaring,
   gluttonous, gouging, great, gross, grossly overpriced, high,
   hyperbolic, hypertrophied, ill-got, ill-gotten, immoderate,
   immoral, incontinent, indefensible, indirect, inexcusable,
   inexpiable, inflationary, inordinate, insidious, intemperate,
   intense, intolerable, irremissible, keen, monstrous, not kosher,
   out of bounds, out of sight, out-and-out, outrageous, outright,
   overbig, overdeveloped, overgreat, overgrown, overlarge, overmuch,
   overpriced, overweening, perfect, piercing, positive, precious,
   preposterous, profound, prohibitive, pronounced, proper,
   questionable, rank, regular, rigorous, rotten, rough, severe,
   shady, shameless, sharp, shattering, shifty, shocking, sinister,
   skyrocketing, slippery, spiraling, splitting, stark, stark-staring,
   steep, stick-at-nothing, stiff, superlative, surpassing,
   suspicious, the veriest, thorough, thoroughgoing, too much, total,
   tough, towering, tricky, unallowable, unbearable, unbridled,
   unchristian, uncivilized, unconscienced, unconscientious,
   undeniable, underhand, underhanded, undue, unequivocal, unethical,
   unforgivable, ungodly, unholy, unjust, unjustifiable, unmeasurable,
   unmitigated, unpardonable, unprincipled, unqualified, unreasonable,
   unrelieved, unrestrained, unsavory, unscrupulous, unspoiled,
   unstraightforward, unwarrantable, unwarranted, usurious, utter,
   vehement, venomous, violent, virulent, wicked, without remorse,
   without shame