The Collaborative International Dictionary of English v.0.48:

Tenable \Ten"a*ble\ (t[e^]n"[.a]*b'l), a. [F. tenable, fr. tenir
   to hold, L. tenere. See Thin, and cf. Continue,
   Continent, Entertain, Maintain, Tenant, Tent.]
   Capable of being held, maintained, or defended, as against an
   assailant or objector, or against attempts to take or
   process; as, a tenable fortress, a tenable argument.
   [1913 Webster]

         If you have hitherto concealed his sight,
         Let it be tenable in your silence still. --Shak.
   [1913 Webster]

         I would be the last man in the world to give up his
         cause when it was tenable.               --Sir W.
                                                  Scott.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tenable
    adj 1: based on sound reasoning or evidence; "well-founded
           suspicions" [syn: tenable, well-founded]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "tenable":
   OK, acceptable, admissible, agreeable, all right, alright,
   arguable, believable, colorable, conceivable, condonable, credible,
   creditable, defendable, defensible, excusable, fiduciary,
   good enough, imaginable, impregnable, justifiable, maintainable,
   okay, passable, plausible, possible, rational, reasonable,
   reliable, secure, supportable, trustworthy, trusty,
   unexceptionable, unimpeachable, unobjectionable, unquestionable,
   viable, vindicable, warrantable, workable, worthy of faith