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.
The Spanish rehusar — literally, “refuse” — sounds odd to English ears: it’s the same word, but the ‑f- became an ‑h-. Huh?
This is explained via the pattern of Latin words that began with an f- tended to turn into an h- in Spanish and only in Spanish. See famine/hambre, and huir/fugitive for example.
Refuse and Rehusar follow the same pattern. Both come from the Latin refundere — from which we also get the English, refund. They are all ways of giving back.
This f‑to‑h pattern usually happens with the first letter of the word. But here it is the first letter of the second syllable — because the re- is of course the standard prefix so it didn’t effect the sound pattern change.
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!
Hervir (Spanish for, “to boil”) comes from the Latin fervere (“to be hot, burn, boil”).
The best part: from this same root, we also get the English… fever!
This is thus another example of the pattern where Spanish lost the initial F and replaced it with the (unspoken) “H”: Hoja-Foliage, Huir-Fugitive, etc.
The Spanish huso (“spindle” — what Cinderella uses to weave!) comes from the Latin for the same: fusus.
The transition is clear when we remember that the initial F in Latin usually turned into an “h” in Spanish: fig vs higo, for example. Or herir vs interfere, for another.
From the same Latin root fusus, we also get the English… fuse. Why? Well, look at the shape: an old-school spindle looks like a big fuse!
Thus, we can see the f‑s of fuse map clearly to the h‑s of huso.