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]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "unconstitutional":
   actionable, against the law, anarchic, anarchistic, anomic,
   black-market, bootleg, chargeable, contraband, contrary to law,
   criminal, felonious, flawed, illegal, illegitimate, illicit,
   impermissible, irregular, justiciable, lawless, nonconstitutional,
   nonlegal, nonlicit, outlaw, outlawed, punishable, triable,
   unallowed, unauthorized, under-the-counter, under-the-table,
   unlawful, unofficial, unstatutory, unwarrantable, unwarranted,
   wrongful



Bouvier's Law Dictionary, Revised 6th Ed (1856):

UNCONSTITUTIONAL. That which is contrary to the constitution.
     2. When an act of the legislature is repugnant or contrary to the
constitution, it is, ipso facto, void. 2 Pet. R. 522; 12 Wheat. 270; 3 Dall.
286; 4 Dall. 18.
     3. The courts have the power, and it is their duty, when an act is
unconstitutional, to declare it to be so; but this will not be done except
in a clear case and, as an additional guard against error, the supreme court
of the United States refuses to take up a case involving constitutional
questions, when the court is not full. 9 Pet. 85. Vide 6 Cranch, 128; 1
Binn. 419; 5 Binn. 355; 2 Penna 184; 3 S. & R. 169; 7 Pick. 466; 13 Pick.
60; 2 Yeates, 493; 1 Virg. Cas. 20; 1 Blackf. 206 6 Rand. 245 1 Murph. 58;
Harper, 385 1 Breese, 209 Pr. Dee. 64, 89; 1 Rep. Cons. Ct. 267 1 Car. Law
Repos. 246 4 Munr. 43; 5 Hayw. 271; 1 Cowen, 550; 1 South. 192; 2 South.
466; 7 N H. Rep. 65, 66; 1 Chip, 237, 257; 10 Conn. 522; 7 Gill & John. 7; 2
Litt. 90; 3 Desaus. 476.