The Spanish celoso and the English for the same, jealousy, come from the same Greek root: zelos.
But how did this happen? They should so different!
The answer is that the Latin (and Greek) words with a -ch- sound and variations (like -sh-, the soft -j-, -z-, etc) usually turned into the hard, guttural, throat-cleaing -j- sound in Spanish. Think about sherry and jerez, for example, or quash and quejar, or soap and jabón.
Thus, the c-l-s of celoso maps to the j-l-s of jealous.
Curiously, the ancient Greek form — zelos — meant jealousy, but in the more positive sense of enthusiasm and friendly rivalry. In a word: zeal — which also comes from the same root!
The Latin sounds for “sh” — and similar variations, like “ch” and “ss” — became a “j” sound in Spanish.
These sh/j sounds were often spelt with a “x” in old Spanish; and sherry itself is named after the town it first came from, Xeres, which is near Cordova.
One of our favorite patterns of sound change between English and Spanish is the sh/j shift: under the influence of arabic, many words that had a “s” or “sh” or “sy” or “ch” sound in Latin, started to be pronounced with the throat-clearing sound and written with a “j”. See sherry/jerez and chess/ajedrez or syrup/jarabe, for example.
Another example of this pattern is the Spanish word for “juice”, jugo. It comes from the Latin succus meaning, “juice” (particularly sap, or juice from plants).
From this Latin root succus we also get the English… suck.
Yes, if it sucks — it is juicy! Literally!
We can see the mapping in the s-c to j-g mapping. The “c” and “g” sounds are similar and often interchanged.
Interestingly, in Spain they do not say jugo to mean “juice”; instead, they say… suco. Suco, funnily enough, also comes from the same root of succus. It is just the variation that never underwent the arabic “j” transformation.
From the same root we also get the English succulent, although we do not get the superficially similar English juice, which comes from the Latin ius, meaning, “sauce.”
The “sh” sound — often represented in writing as an “x” — transformed in all different ways to the “j” letter (and the accompanying mouth-clearing sound, influenced by Arabic) as late Latin turned into Spanish. See lots of examples: sherry/jerez, for example.
Here’s another: the common Spanish word, dejar, meaning, “to leave to the side” or “to put down” or to “put away” or to just “let go.”
Dejare comes from the Latin laxare, meaning, “to loosen”. From this same root, we get a few English words — which did not go through the x-to-j transformation Spanish did including:
The Spanish eje for “axle” comes from the Latin for the same, axis. The English axle comes from the same common ancestor as the Latin axis, the proto-indo-european root *aks– also meaning the same.
The Spanish eje is easy to understand if we remember that most of the x/sh/ch sounds in Latin and the ancient languages became the throat-clearing -j- sound in Spanish. Thus, the e-j of eje maps to the a-x of axle pretty clearly.
It’s interesting how such a simple word has remained mostly unchanged for tens of thousands of years. Perhaps, the axle is one of the most fundamental discoveries in human history. It is, after all, what led to the wheel, which led to… civilization.