The Collaborative International Dictionary of English v.0.48:

Mattock \Mat"tock\, n. [AS. mattuc; cf. W. matog.]
   An implement for digging and grubbing. The head has two long
   steel blades, one like an adz and the other like a narrow ax
   or the point of a pickax.
   [1913 Webster]

         'T is you must dig with mattock and with spade. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mattock
    n 1: a kind of pick that is used for digging; has a flat blade
         set at right angles to the handle

Easton's 1897 Bible Dictionary:

Mattock
   (1.) Heb. ma'eder, an instrument for dressing or pruning a
   vineyard (Isa. 7:25); a weeding-hoe.

     (2.) Heb. mahareshah (1 Sam. 13:1), perhaps the ploughshare or
   coulter.

     (3.) Heb. herebh, marg. of text (2 Chr. 34:6). Authorized
   Version, "with their mattocks," marg. "mauls." The Revised
   Version renders "in their ruins," marg. "with their axes." The
   Hebrew text is probably corrupt.