Fervor is really just an intense passion heating up. Thus we shouldn’t be surprised that it comes from the Latin root fervere (“to boil”), from which we get the Spanish for the same (“to boil”), hervir.
The seemingly unrelated words are connected through the common transformation of Latin words beginning with an f- into an h- in Spanish, such as fig and higo, and fable and hablar.
Thus, the f‑r-v of fervor maps to the h‑r-v of hervir.
“Fig” comes from the Latin “Ficus” — obvious enough!
But, curiously, the Spanish word is “Higado”. Huh?
This is just a simple example of the Initial F to H pattern. In lots of Latin words, the first F became an H when Latin evolved into Spanish. Think fact/hecho or hablar/fable.
An easy way to figure out what an H- word in Spanish is: change the initial H to an F and see what English word sounds similar.
The Spanish hablar (“to talk”) comes from the Latin fabulare, as we’ve previously discussed. The initial F- turned into an H‑, as happens only in Spanish (think fig vs higo.)
From the same root, however, also comes the English ineffable, that SAT word meaning “unable to be described in words.” So, ineffable literally means “without” (in-) and “speaking” (fabulare).
We see the h‑b-l of hablar map to the (in-)f‑b-l of ineffable quite clearly!
The Initial F, followed by a vowel, disappears: So, “hoja”, meaning “leaf” (in all senses: the autumn trees, the piece of paper) is thus, from the same Latin root as “foliage”, the green plant leaves!
If he is fuming, he is smoking — literally. And it is, subtly, the same word in Spanish.
“To fume” comes from the Latin root fumus (“smoke”) from which we also get the common Spanish word for “smoke”, humo. But they don’t sound alike, so how are they related?
The Spanish humo is a great example of the pattern of the Initial F turning into an H in Spanish, alone among the languages of the world. Many Latin words that began with an F, and come to us in English through the Latinate F form, became the equivalent word but with an H- in Spanish. Take hermano and filial, for example. Or fact and hecho.
Other English words from the same root fumus include fumigation (ahhhh!) and the less common fetid. Fetid is a dirty, Shakespearean word, after all.