Ah, your friend's Italian nonna sounds like a real character! "Mannagg" is actually short for "mannaggia", which is a way of expressing frustration or annoyance in Italian. It's kind of like saying "darn" or "dang" in English. As for "stunad", that's a slang term in Southern Italian dialects that roughly translates to "stupid" or "dumb". So when your friend's nonna says "Mannagg, are you stunad?", she's basically saying "Darn it, are you stupid?" or "Dang it, are you dumb?" It's a bit of tough love, Italian-style. Hope that helps, paisan!