The Collaborative International Dictionary of English v.0.48:

Tenability \Ten`a*bil"i*ty\ (t[e^]n`[.a]*b[i^]l"[i^]*t[y^]), n.
   The quality or state of being tenable; tenableness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tenability
    n 1: the quality of being plausible or acceptable to a
         reasonable person; "he questioned the tenability of my
         claims" [syn: reasonableness, tenability,
         tenableness]