From ToEE

No silly, it's not a hammer made out of hay. It's a pole-arm. The DM is being nice, and allowing clerics to use them. However, they have the regular disadvantages of a pole-arm, and clerics need to have the pointy bit filed off.

A good discussion of polearms can be found at http://hea-www.harvard.edu/%7Efine/Fun/polearms.html .