Source : Webster's Revised Unabridged Dictionary (1913)

Shale \Shale\, n. [AS. scealy, scalu. See {Scalme}, and cf.
   {Shell}.]
   1. A shell or husk; a cod or pod. ``The green shales of a
      bean.'' --Chapman.

   2. [G. shale.] (Geol.) A fine-grained sedimentary rock of a
      thin, laminated, and often friable, structure.

   {Bituminous shale}. See under {Bituminous}.

Shale \Shale\, v. t.
   To take off the shell or coat of; to shell.

         Life, in its upper grades, was bursting its shell, or
         was shaling off its husk.                --I. Taylor.

Source : WordNet®

shale
     n : a sedimentary rock formed by the deposition of successive
         layers of clay