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"