This powerful melee weapon is dropped by the secret giant glowing ghoul boss in Docks Shipyard during Chapter 2.
It's a very powerful weapon for chapter 2 and remains very good even in chapter 3. It has powerful critical damage, and with every critical hit, the enemy is thrown back very far. Despite these characteristics, in Chapter 3, there are better melee weapons.
To get it when you're on the Docks Shipyard, check all of the alleyways north of the large area of radioactivity until you find a couple of glowing ghouls standing behind an unscalable fence. They cannot be reached by standard weapons, but you can hit them with explosives or other weapons like incendiary mortar. Kill these ghouls and the giant glowing ghoul will spawn just outside the alley's entrance. After that, you need just kill him and he drops the Slugger.