The Collaborative International Dictionary of English v.0.48:

Punishable \Pun"ish*a*ble\, a. [Cf. F. punissable.]
   Deserving of, or liable to, punishment; capable of being
   punished by law or right; -- said of person or offenses.
   [1913 Webster]

         That time was, when to be a Protestant, to be a
         Christian, was by law as punishable as to be a traitor.
                                                  -- Milton.
   [1913 Webster] -- Pun"ish*a*ble*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

punishable
    adj 1: liable to or deserving punishment; "punishable offenses"
    2: subject to punishment by law; "a penal offense" [syn:
       penal, punishable]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "punishable":
   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, triable, unallowed,
   unauthorized, unconstitutional, under-the-counter, under-the-table,
   unlawful, unofficial, unstatutory, unwarrantable, unwarranted,
   wrongful