Hello Gene, I am afraid I'm going to explain something many people clearly do not know. But, as a "Costa" myself, I feel you deserve to know. The origin of the name Costa or the "costas" as in "back" relates to the word costelas, or meaning ribs in English. I am attaching the coat of arms for the name "Costa" which can be seen in the National Palace of Sintra, in the greater Lisbon. You can see the ribs represented there. And I'm also providing a Wikipedia article https://en.wikipedia.org/wiki/Costa_(surname) where it can be read "In Italy, Portugal, Galicia and Catalonia it is derived from the Latin word costa, 'rib'." That's why we say "costas," because our backs are composed by more than one rib. Have a nice day.

We say "costas" because we mean both "flanks". I know, it's not so common in other languages but that's how it is.

I THINK THAT IS TO DIFFERENCIATE BECAUSE "COSTA" IS THE WAY TO TALK ABOUT THE BORDER WITH THE SEA.

hahaha Good question! Portuguese can be tricky, but in the end there is always a reason why things are how they are haha like the guys here explained. :) If you are intrigued by some brazilian words or expressions I invite you to take a look at my profile to book a lesson with me! I love studying the history behind some particularities of our language and exchanging it with my students! Um abraço!

I agree with Agatha, but it is just how portuguese is I am afraid 😅 if you say "a minha costa" it definitely sounds like your have a coast line if that makes sense 🤣