The Collaborative International Dictionary of English v.0.48:

Unwarrantable \Un*war"rant*a*ble\, a.
   Not warrantable; indefensible; not vindicable; not
   justifiable; illegal; unjust; improper. --
   Un*war"rant*a*ble*ness, n. -- Un*war"rant*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unwarrantably
    adv 1: in an unwarrantable manner or to an unwarranted degree;
           "in this painting, the relationship of the upper part of
           the body to the lower is uneasy and the right thigh seems
           unwarrantably stressed"