A post pounder is a tool used for driving metal fence and sign posts into the ground. It consists of a heavy iron pipe which is closed at one end and has handles welded onto the sides.
The open end is put over the top of the post to be driven and moved up and down. The closed end hammers on the top of the post and the large mass aids in sinking it into the ground.
Manual post pounders tend to be around 20 lbs, but weight varies with model types. They can be dangerous to handle if raised above the head. Incidentally catching the bottom of the pounder on the top of the post can result in serious injury to the head if the pounder continues to pivot and fall downward.
Hearing protection is advised due to the loud sound produced. It would be wise to wear a hard hat.