Fun fact: The Proto-Indo-European ancestor language has (or rather had) two words for a fart. One which is the ancestor of “fart” and another for which no descendant exists in English. That word was for silent farts. Curiously though, that word is the ancestor of the French word for fart, “pet”.

If it had made it to English, it would probably be something like “fast” - and it’s immediately obvious why it doesn’t exist, because there’s an important word with a completely different meaning occupying that niche.

Vowels are somewhat negotiable though, so we could opt for the next best, which is “fest”, which can’t have existed or else we probably wouldn’t have adopted the current meaning from French (and/or German).

“Fust” on the other hand, well, that’s interesting. It’s a word that means a musty smell and supposedly derives from a completely different root, but the similarity and onomatopoeia make me wonder…

Edit: Tense agreements. I grammar good