What does "FOW" stand for in Just Cause 3?

Flames Of War. It's said a few times by Ravello, he's referencing his faction's capabilities. Each time you take a region's flame of war, the rebels work it into something Rico can use in the field.

Although the actual definition is never revealed within the game, a weapon that is classified as a FOW is a weapon that has been infused with Bavarium.

Any weapon infused with Bavarium is classed as a "FOW" weapon by the Medici Military. The exact definition of "FOW" is never revealed in the game. It is sometimes pronounced as an abbreviation, whilst at other times it is pronounced as an acronym. The letter "W" likely stands for Weapon.