The Collaborative International Dictionary of English v.0.48:

Actionable \Ac"tion*a*ble\, a. [Cf. LL. actionabilis. See
   Action.]
   That may be the subject of an action or suit at law; as, to
   call a man a thief is actionable.
   [1913 Webster]

WordNet (r) 3.0 (2006):

actionable
    adj 1: affording grounds for legal action; "slander is an
           actionable offense"

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "actionable":
   against the law, anarchic, anarchistic, anomic, applicable,
   authorized, black-market, bootleg, causidical, chargeable,
   competent, constitutional, contraband, contrary to law, criminal,
   felonious, flawed, illegal, illegitimate, illicit, impermissible,
   irregular, judicial, juridical, just, justiciable, kosher, lawful,
   lawless, lawmaking, legal, legislative, legit, legitimate, licit,
   litigable, litigant, litigatory, litigious, nonconstitutional,
   nonlegal, nonlicit, outlaw, outlawed, punishable, rightful,
   sanctioned, statutory, triable, unallowed, unauthorized,
   unconstitutional, under-the-counter, under-the-table, unlawful,
   unofficial, unstatutory, unwarrantable, unwarranted, valid,
   within the law, wrongful