Given its name, (cut) ability, and location (a mirelurk butchery), it is likely this weapon was originally used for literally tenderizing mirelurks.
The Tenderizer is a unique sledgehammer. It deals 50% more damage and is 50% more durable than a regular sledgehammer. It was supposed to have an effect that damaged enemy armor with each hit, but the effect is not active; see Notes for more details.
Located in Anchorage Memorial in the utility room with the broken door. To find the door, enter the Memorial Service Entrance on the east side of the memorial. At the junction, turn left and it's the second door on your right. The door can be fixed by finding the door component, but note that you need a Lockpick or Science level of 50 to be able to open the safe in which the door component is stored in the top level of the building. (You can look behind the Nuka-Cola dispenser to obtain the pass code for the computer in the same room, which will then allow you to unlock the safe.) A Repair skill of 35 or higher is required to fix the door with the help of the component. Without the component a repair skill of 95 is required.
The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
The Tenderizer was meant to have an effect which damages enemy armor on each hit by 5 points. However, for some reason its Object Effect is actually missing from the weapon itself, so The Tenderizer does not have any additional bonus. On PC it can be fixed with the G.E.C.K. (The Tenderizer's Object Effect, known as EnchWeapUniqueTenderizer is there in the Object Effect list, it just isn't selected for The Tenderizer).
End of information based on Fallout 3 cut content.