I believe this phenomenon is called 'assimilation': https://en.wikipedia.org/wiki/Assimilation_(phonology)#Anticipatory_assimilation_to_an_adjacent_segment
So, for an earlier exercise they used "mongrel" for hundaĉo, which I strongly disagree with. But in this case fiulo really ought to be something like "villain" or "scoundrel." There are a truckload of such synonyms. I really don't understand the inconsistency. Translating it as "bad person" defeats the purpose of the sentence.
It is funny that Esperanto chose this particle to express contempt, with so many Latin and Romance languages' etymons. The word for “to trust“ in Latin is confido, as in con + fi + do ≡ “with” + “trust” + “give”. This exercise in recreational etymology yields the fun fake fact that maybe Zamehof was such a good-hearted person that he thought even the most contemptible people are trust-deserving. (Or is it all the other way around?)
What I find funny is that this is the first time I've ever seen the Latin fido broken into two words. Partly because the Latin fi is the (if I have recalled this correctly) passive form of facio = "to make, construct, build, etc." whereas fido = "to trust, confide, rely upon, etc." is exactly as it exists in Esperanto. But yes, the Latin do does = "to give up, hand over, deliver, etc." So I'm certain that some old Roman comedians were making wordplay about "faithfully handing over built stuff" back when.
But the Esperanto fi has a different history: the (olde) English fie which seems to have come from the French fi, but we also find the Swedish (and Norwegian) fy, and the Greek (I don't have that font here) phy, the Russian (best I can approximate is) Tbøy, the Lithuanian fe, the Latin fu, and the Volapuk fi - all of which seem to be associated with disparagement on some level.
Especially the Volapuk.
Wow! I did not expect this! I suspected that fi had something to do with the French, even though I had no way to check it, but all the other coincidences come as a complete surprise to me. Is there any online source of reliable etymology other than Wiktionary?
Anyway, my little joke was not worthy of your deep analysis and your time, but thank you very much.
I don't know about on-line, but one of my very appreciated books is Konciza etimologia vortaro by André Cherpillod. I also referenced "An Elementary Latin Dictionary" (which is very thick) by Charlton T. Lewis. Near as I can determine neither book has made it on-line yet.
And yes, I figured that your post was ŝercema, which is why I attempted to respond in kind.
If you aren't having fun here, then you are doing it wrong. :D
Oh, dear Duo. Here you are teaching us bad words.
fi! I- Interj., esprimanta malestimon k abomenon
-aĉ/1. I- Suf. esprimanta senvalorigon (pro malbela aspekto, maltaŭgeco, malbona kvalito aŭ karaktero ks)
"Aĉ" comments on poor quality and at least comes with some pity and recognizes subjectivity, "fi" comments on poor moral character and is completely contemptuous and presents itself as an objective judgement. The best English equivalents are often taboo, and I find it really interesting that "fi" makes for some of the strongest invective.
So, and apologies for the English, "homaĉo" is a poor little ❤❤❤❤ but "fiulo" is a f****ng worthless ❤❤❤❤. Be careful with it.
The other odd thing is that "fi" is not too terribly taboo. It's okay to call out badness, especially badness of actions. Just remember that "fi" is how you scold a dog, and "ne" is how you disagree with someone you have at least a little respect for.