A multipurpose exclamation. It can denote surprise, shock, exasperation… I’m probably missing a bunch of uses here, but these are three of the main ones - it’s a very versatile exclamation.
Literally, Nossa means “Our”.
"Um, wtf?" you may ask. This one actually makes sense: Nossa in this context is a shortening of Nossa Senhora (Our Lady), a reference to the Virgin Mary.
It was likely considered a nasty swear way back in the day (a la “My God!”, which we also use - Meu Deus!). Nowadays it holds no hint of being a swear anymore.
Variants of Nossa! include (but are certainly not limited to):
Nossa Senhora! - Our Lady!
Nossa Mãe! - Our Mother!
Ave Maria! - Hail Mary!
Santa Maria! - Saint Mary!
Vixe! - Virgin!
In English people would say “Gee” instead of “Jesus”. Like Gee, vixe is a way of referring to a holy person without using their proper name/title (the proper word for “virgin” in Portuguese is virgem - it might sound familiar due to its Latin root: Virgo).
I doubt many Brazilians even realize that’s what vixe (pronounced “vee-shee” or “vee-shh”) even refers to (I certainly didn’t until I actually stopped to think about it); it’s just a popular exclamation nowadays.
Vixe Maria! - Virgin Mary!