Wordnet 3.0
ADVERB (1)
1. 
 with good reason; 
- Example: "he is justifiably bitter"
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):
justifiably
    adv 1: with good reason; "he is justifiably bitter" [ant:
           inexcusably, unjustifiably]