This is the 352nd most frequent Portuguese word.
costa
"Coast" or "back" (depending on context).
Here, 'costa' refers to the geographical feature corresponding to the coastal area.
A costa do Brasil é extensa e cheia de praias.
The coast of Brazil is extensive and full of beaches.
In this sentence, 'costas' means the back of the body.
Levei minha mochila nas costas durante toda a viagem.
I carried my backpack on my back throughout the journey.
In this context, 'costa' describes a specific region adjacent to a body of water within a country.