The Collaborative International Dictionary of English v.0.48:

Defensible \De*fen"si*ble\, a. [Cf. F. d['e]fensable, LL.
   defensabilis, defensibilis. See Defense, and cf.
   Defendable.]
   1. Capable of being defended; as, a defensible city, or a
      defensible cause.
      [1913 Webster]

   2. Capable of offering defense. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

defensible
    adj 1: capable of being defended [syn: defendable,
           defensible]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "defensible":
   admissible, allowable, balanced, condonable, defendable, deserved,
   dispensable, due, equitable, even, evenhanded, excusable,
   exemptible, expiable, fair, fair and square, fit, forgivable, good,
   inoffensive, just, justifiable, justified, lawful, legal,
   legitimate, level, meet, meet and right, merited, pardonable,
   proper, reasonable, remissible, right, right and proper, rightful,
   square, tenable, unobjectionable, venial, vindicable, warrantable,
   warranted