The Collaborative International Dictionary of English v.0.48:

Justifiable \Jus"ti*fi`a*ble\, a. [Cf. F. justifiable. See
   Justify.]
   Capable of being justified, or shown to be just.
   [1913 Webster]

         Just are the ways of God,
         And justifiable to men.                  --Milton.

   Syn: Defensible; vindicable; warrantable; excusable;
        exculpable; authorizable. -- Jus"ti*fi`a*ble*ness, n.
        -- Jus"ti*fi`a*bly, adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

justifiable
    adj 1: capable of being justified