Someone asked this on Reddit, and I tried to dive into the lore but came up with nothing. I did find that the kana used to spell it in the original Japanese is just the English word "foul" (ファウル or "fauru") which was no help at all.

Normally if a spell is the same in the original but has different names in English translations, you can figure that out that way, but the only spell I can find that uses that word in any FF game is Foul Waters ("fauru wota"), a blue mage spell that does water damage and helps resist silence in FFXI. In French, the term used for it is Infect, which suggests "unclean" or "diseased" or "toxic."

The person who asked the question on Reddit joked that the Black Mage was probably just swearing at someone, using the effect of foul language. I don't think they were serious, but since it operates under the effect of Polyglot (speaking many languages or literally "many tongues") that's honestly as good a theory as any.

My own guess is that's Bio/Virus, but they had used both names already for things in the game. Black Mages have always had access to poison, and at later levels they attacked with disease. In Final Fantasy III, IV, V, and VI it takes the form of giant bacteria or viruses attacking the target. I am thinking that since they'd used the name already, they came up with a new name for Black Mage's traditional "hit them with a poison/disease" attack.