: The point-and-click adventure game Adventures of Fatman ?

: The iconic mini-nuke launcher from the Fallout series (Fallout 3, 4, or 76)?

That depends on which "Fatman" you're looking for, as it's a popular name in games and movies. Could you clarify if you mean:

: The bomb-obsessed, roller-skating villain from Metal Gear Solid 2: Sons of Liberty ?

: The 2020 action film Fatman starring Mel Gibson as a rowdy Santa Claus?