The Collaborative International Dictionary of English v.0.48:

Tenableness \Ten`a*ble*ness\, n.
   Same as Tenability.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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