From pois (“so, then”) + não (“not”). Probably originally intended as: "so ... not ...?", implying either "so, how could I not serve you?" or "so, how could I not do what you asked?", depending on the context.
- (Brazil, formal) Can I help you? (used when attending customers)
- (Brazil, formal) yes; of course (used to confirm a request)
- “Eu gostaria de uma salada.” — “Pois não, senhor.”
- “I would like a salad.” — “Yes, sir.”
Don't confuse with pois sim (“used as a sarcastic agreement of something that is obviously to be disagreed: yeah, right!, yeah, sure!”).