To unlock this you will need to go to your friend Nika's house [the building to the left of the Lolita shop with the planter on the window].
Go upstairs to her room and you'll notice a hammer laying on the ground next to her experiment. In order to take it, you'll need to speak to her about it.
After she agrees to let you use it - Run back to the butcher shop and in your room there will be a piggy bank next to a saw blade.
Simply equip the hammer and smash it.