The Collaborative International Dictionary of English v.0.48:

Unreasonable \Unreasonable\
   See reasonable.

The Collaborative International Dictionary of English v.0.48:

Unreasonable \Un*rea"son*a*ble\, a.
   Not reasonable; irrational; immoderate; exorbitant. --
   Un*rea"son*a*ble*ness, n. -- Un*rea"son*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unreasonable
    adj 1: not reasonable; not showing good judgment [ant:
           reasonable, sensible]
    2: beyond normal limits; "excessive charges"; "a book of
       inordinate length"; "his dress stops just short of undue
       elegance"; "unreasonable demands" [syn: excessive,
       inordinate, undue, unreasonable]

Moby Thesaurus II by Grady Ward, 1.0:

200 Moby Thesaurus words for "unreasonable":
   a bit much, abandoned, absonant, absurd, arbitrary, bigoted, blind,
   boundless, brainless, capricious, contradictory,
   contrary to reason, cranky, crazy, crotchety, cutthroat, egregious,
   enormous, exacting, exaggerated, excessive, exorbitant,
   extortionate, extravagant, extreme, extremist, fabulous,
   fallacious, fanatic, fanciful, fancy, fantasied, fantastic,
   fatuous, faulty, flaky, flawed, foolish, freakish, gigantic,
   gluttonous, gouging, grossly overpriced, haggard, harebrained,
   high, humorsome, hyperbolic, hypertrophied, idiotic, ill-advised,
   ill-considered, ill-contrived, ill-devised, ill-gauged, ill-judged,
   illogical, imbecilic, immoderate, impolitic, impractical, improper,
   imprudent, inadvisable, inappropriate, inapt, inauthentic,
   inconclusive, incongruous, inconsequent, inconsequential,
   inconsiderate, inconsistent, incontinent, indefensible, indiscreet,
   inequitable, inexcusable, inexpedient, inexpiable, inflationary,
   injudicious, inordinate, insane, insensate, intemperate, invalid,
   irrational, irremissible, kinky, laughable, loose, ludicrous, mad,
   maggoty, mindless, misadvised, misguided, monstrous, moody,
   moronic, motiveless, myopic, nonrational, nonscientific,
   nonsensical, not following, notional, out of bounds, out of sight,
   outrageous, overbig, overdeveloped, overenthusiastic, overgreat,
   overgrown, overlarge, overmuch, overpriced, overreligious,
   overweening, overzealous, paralogical, peremptory, perfervid,
   petulant, preposterous, prohibitive, quirky, rabid, reasonless,
   reckless, ridiculous, self-annulling, self-contradictory,
   self-refuting, senseless, short-sighted, shortsighted, silly,
   skyrocketing, sophistic, spiraling, steep, stiff, stupid,
   temperamental, thoughtless, too much, ultrazealous, unadvised,
   unallowable, unapt, unauthentic, unbefitting, unbridled,
   uncalled-for, unconnected, unconscionable, unconsidered,
   undiscerning, undue, unequal, unfair, unforeseeing, unforgivable,
   unjust, unjustifiable, unjustified, unlawful, unpardonable,
   unperceiving, unperceptive, unphilosophical, unrealistic,
   unreflecting, unreflective, unrestrained, unscientific, unseeing,
   unsensible, unsound, unsuitable, unthinking, unthoughtful,
   unwarrantable, unwarranted, unwise, usurious, vagarious, vagrant,
   wanton, wayward, whimsical, wild-eyed, wild-looking,
   without reason, witless, wrongful, zealotic