In Portugeuse "thank you" is obrigado/obrigada. Is which one used determined by who is being thanked or who is doing the thanking?

"It is very easy to say thank you in Portuguese:

* Men say Obrigado
* Women say Obrigada

Why is that?

The first thing to have in mind is that in Portuguese adjectives vary in gender to agree with the noun they are modifying....Obrigado comes from "I feel obliged". That is, when someone does me a favor, I feel grateful and obliged to that person. So the word Obrigado will vary in gender to agree with who is feeling so.

This is a simple and easy rule to memorize. Guys always say Obrigado. Girls always say Obrigada. This is also one of those rules that people don't know about or forget. I can assure you that you will hear Brazilian women saying "Obrigado". They should not. And now you know why."

http://streetsmartbrazil.blogspot.com/2008/12/obrigado-and-obrigada-thank-you.html