Unreasonable, a.
Not reasonable; irrational; immoderate; exorbitant. -- Un*rea"son*a*ble*ness, n. -- Un*rea"son*a*bly, adv.
Shades of meaning
Fernald has no entry of its own for unreasonable; he lists it under absurd.
Where he writes about it:
Anything is irrational when clearly contrary to sound reason, foolish when contrary to practical good sense, silly when petty and contemptible in its folly, erroneous when containing error that vitiates the result, unreasonable when there seems a perverse bias or an intent to go wrong. — from absurd