We’ve already discussed desayuno (“breakfast”): breakfast is the break-fast, just like des- (“anti”) ayuno (“fast”)!
However, there’s an interesting addition to the story: dinner.
The English dinner originally comes from the French for breakfast, which is almost the same as the Spanish. Both are from Latin and meant the same: desjunare. Thus, we can see over time that the Latin for break-fast (dis– + ieiunus) became “breakfast” in both French and Spanish and then, via the French transformed into the English dinner while remaining with the same meaning in Spanish.
Therefore, we can see the d-(s)-n of desayuno map to the d-n of dinner.
But all of this suggests a question: how did breakfast (the first meal of the day) turn into dinner (the last meal of the day)?
Easy: breakfast kept on getting later and later — until it was dinner!
We see this vestige of the old usage in England, where dinner is sometimes used to refer to “lunch”–and the night-time meal that Americans call dinner is still sometimes called… supper.