The Spanish for “to teach,” enseñar comes from the Latin insignare (“to mark”). From the same Latin root, we get the English to sign — signing, after all, is making your mark upon a paper!
But how did signing turn into teaching, in Spanish? Well, think about the English expression… to make a mark on someone. A great teacher truly leaves a lasting mark on you — literally.
The s‑ñ of enseñar maps to the s‑gn of “sign,” with the ñ turning into a gn in English, as it commonly does.
Tamaño (Spanish for “size,” in the size of, “what is your pants size?”) comes from the Latin tam — magno, that is, “so — great” (“great” in the size of “big”). Tam is the Latin for “so” or “very” from which we get the Spanish tan.
To even measure is thus to imply that… you are big! So great! If you’re small, after all, you don’t even need to measure it!
Magno (Latin for “great” or “big”) gives us the English… magnificent. But, curiously, the -gn- turns into the ñ as Latin evolved into Spanish. Thus tan — magno became tamaño. We see this gn to ñ pattern in many words, such as cognate / cuñado.
The English for eager-to-fight, pugnacious, contains the ‑gn- pattern inside it: a give-away to the pattern that ‑gn- words in Latin turned the ‑gn- into a ‑ñ- in Spanish yet remained the same into English.
Therefore, pugnacious maps perfectly to puñal, the Spanish for… “dagger.” It makes sense that “dagger” and “eager to fight” come from the same root, after all. And that root, in this case, is the Latin pugnare, meaning, “to fight.”
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.