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!
Quejar, Spanish for “to complain” doesn’t seem related to any English equivalent.
But upon closer look, it is a first cousin of both quash and squash.
All come from the Latin quassare, meaning, “to shatter.”
The relationship is easy to see if we remember that the Spanish ‑j- sound used to be the Latin ‑s- sound (and many variants, like ‑ss‑, ‑si‑, ‑sy‑, ‑sh‑, ‑ch‑, etc).
Thus, the qu‑j for quejar maps to the qu-sh of quash and the sq-sh of squash.
Complaining, it seems, is a form of quashing (squashing?) your opponent!
Perejil and its English version parsley sound very different. But they are, actually, etymologically the same word.
They sound different because often the ‑s- and ‑sh- sounds in Spanish turned into the letter ‑j- with the Arabic throat clearing sound as a pronunciation. Thus, the p‑r-j‑l of perejil maps exactly to the p‑r-s‑l of parsley.
The Spanish bajo, for “low”, sounds unlike the similar words in English.… except for base.
Think about base as the core foundation or support — the lowest thing holding everything else up — or even in the old Shakespearean sense of “vile”, “the basest weed” — the connection makes much more sense.
Both come from the Latin basis (meaning, “foundation”) — from which we also get the same English, basis.
And think of the bass cleff in music, for the lower notes, as well.
The surprising connection is explained easily when we understand that a lot of sh- and si- and related sounds in Latin turned into j- in Spanish. Thus, the b‑s maps to b‑j almost exactly.
The Spanish for syrup, jarabe, comes from the same root as the English: the Persian/Arabic sharab, which means “a drink, or wine”.
The drastically different (at least superficially) words are explained by the sh- and related (such as, sy- ) sounds changing to the Arabic-sounding j- sound in Spanish — but not English.
Thus, the j‑r-b of jarabe maps to the sy-r‑p of syrup.