How do you say why thank you sir in Brazilian?

Answer 1

Obrigado (if you are male), obrigada (if you are female). Senhor (sir) is only ever used when you are talking to an elder (someone old enough to look like a guaranty kind of elder) or in very formal situations.

Answer 2

In Brazilian Portuguese, you can say "obrigado, senhor" to mean "why thank you, sir."