if Fahluaan is canon of Gardener (referring to Hermaeus Mora, the Gardener of Men, who is also known as the Inevitable Knower) then there must be a related root meaning Garden, which is thought to be Fahlu. Though the -u ending is unusual aside from some single-syllable words, it isn't alone, as the canon word for army is Lahvu (and I don't think it's a compound word from "magic" and "dawn"). Other words ending in -u are a compound ending in -nu.
But.
If the dragon Nahfahlaar is named by the three words Nah Fahl Aar (fury ___ servant), then Fahl has to mean something on its own. Is the -u in Fahlu a possessive suffix? If so, is "garden" actually "our ___"? Fahl could mean several things, such as "plant" or "seed" or "property". It could also just be the verb form of "garden".
I have issues with the word overall, as -aan is a past participle indicator. But that would make the word "Gardened" and the phrase Fahluaan do Jul doesn't translate properly.
Is Fahluaan just an irregular word? Or did Bethesda just throw it out there thinking it made sense lol.