The Collaborative International Dictionary of English v.0.48:

Unconstitutional \Un*con`sti*tu"tion*al\, a.
   Not constitutional; not according to, or consistent with, the
   terms of a constitution of government; contrary to the
   constitution; as, an unconstitutional law, or act of an
   officer. --Burke. -- Un*con`sti*tu"tion*al"i*ty, n. --
   Un*con`sti*tu"tion*al-ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unconstitutional
    adj 1: not consistent with or according to a constitution;
           contrary to the U.S. Constitution [ant: constitutional]