When Pigs Fly is a quest introducing in the next update to Fallout 4 that will give you your hands on improvised weapons including the Piggy Bank Fat Man and the Baseball Launcher.
Fallout 4: Where to Find All the Improvised Weapons | When Pigs Fly Quest
Once you log into Fallout 4, you should receive the When Pigs Fly quest along with the rest of the new missions. If you’re ready to arm yourself with some of the most ridiculous weapons in the Wasteland, read on to learn how to complete the When Pigs Fly quest in Fallout 4.
The first step in the When Pigs Fly quest is to investigate the merchant's workshop east of Kendall Hospital. Go to the location marked on the map and look for the open garage with an NPC named Suggs inside.
You can talk to Suggs or buy items like Shipments of Copper. But to work on the quest, you need to interact with the terminal behind him. Read the Weapons Dealer in Monsignor Plaza document and the quest will proceed to the next step.