This unique-named legendary weapon is found within the massive floating raider encampment, Libertalia. It is located at the top of a lootable wooden crate within the captain's cabin at the top of the compound. The crate is located immediately across the room from the door, beside a burning lantern.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
While the French is not perfectly translated (translating more directly to "The Terrible Gun", with "terribles" unnecessarily pluralized), the name, along with the gun's role as a unique combat shotgun with higher damage but higher recoil, all make it a likely reference to The Terrible Shotgun from Fallout 3.