Nice find! Based on just the one picture I'd say it's a particular style of a large hoe. More pictures are needed of both sides and edgewise for a more certain I.D., because it could also possibly be an adze or a Celt. Whatever it was do to its size it was most likely "long handle" halfted, that is, attached to a long handle like a modern day garden hoe Many large hoes of that general shape and size have been documented.
Again, NICE find!