The Collaborative International Dictionary of English v.0.48:

Fathomless \Fath"om*less\, a.
   1. Incapable of being fathomed; immeasurable; that can not be
      sounded.
      [1913 Webster]

            And buckle in a waist most fathomless. --Shak.
      [1913 Webster]

   2. Incomprehensible.
      [1913 Webster]

            The fathomless absurdity.             --Milton.
      [1913 Webster]