1. (Brazil, especially in the rural areas of the Southeast) why (exclamation of surprise)
2. (Brazil, especially in the rural areas of the Southeast) huh (expresses doubt or confusion)

According to Streets Smart Brazil, 

"There are three explanations for “uai”. The first says it derives from the 1800’s when the English built the railways in Minas, though the idea that Brits frequently said “why” as a catchphrase just doesn’t add up. The second explanation says that at the time of the Mineiro Uprising, the way to enter a conspirator’s hideout was via three knocks and the word UAI, which meant Union, Love and Independence (União, Amor e Independência). The last and, what I consider to be the most likely, is that “uai” is just a mutation of the common Paulista exclamation, “ué”.

An uncomplicated explanation of the term would simply be, “uai é uai, uai!” Agreed, uai is uai…unless, of course, you happen to be an English teacher in Minas Gerais, never knowing if your students are asking “why?” or saying “uai”."