The Collaborative International Dictionary of English v.0.48:

Unsearchable \Un*search"a*ble\, a.
   Not searchable; inscrutable; hidden; mysterious.
   [1913 Webster]

         The counsels of God are to us unsearchable. --Rogers.
   [1913 Webster] -- Un*search"a*ble*ness, n. --
   Un*search"a*bly, adv.
   [1913 Webster]