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