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]