Cuñado, Spanish for “brother-in-law,” comes from the Latin cognatus, from which we get the near-identical English cognate. How can two words so similar mean something so different?
The Latin root cognatus itself came from the roots com– (meaning “together”) and gnasci (meaning “to be born”); thus, literally, “born together.” So, two words that are cognates are — etymologically-speaking — words that are born together. And brothers-in-law are two men who are not brothers but were, in effect at least, born together as well.
Note also that this is an example of the pattern whereby Latin words with a -gn- generally became an ñ in Spanish. Thus the c-gn-t of cognate maps to the c-ñ-d of cuñado.
Sueño (Spanish for “dream”) and insomnia come from the same root: the Latin somnus, meaning, “sleep.”
The evolution is easy to spot if we remember that the -mn- sound in Latin usually transformed into the ñ in Spanish. See damn and daño, for example. Or autumn and otoño as well.
Thus, the s-mn of insomnia maps to the s-ñ of, sueño.
We recently discussed the relationship between dejar and relax, both from the same Latin root, laxare, from the Latin laxus. Other modern words come from these same roots, let’s see…
In Spanish, another interesting word from the same root is lejos, meaning, “far.” This underwent the same sh to j transition documented in the other post. That which is far away, after all, is what we can be relaxed about, what it’s easy to be loose about.
Some additional English words that come from this same root include:
Embassy (and Ambassador) and its Spanish equivalent, Embajada (and Embajador), both come from the same ancestor, the Old French Ambactos.
What is most interesting about these two is that it is an example of the pattern where the -j- sound in Spanish maps to the -sh- sound (and its cousins, like -ss- and -ch-) in English. Remember syrup and jarabe, chess and ajedrez, sherry and jerez, and push and empujar for a few examples.
Thus, the m-b-j of emabajada maps to the m-b-ss of embassy.
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.