Hi there.
I moved to Croatia a few years ago and since I have been here I always see the inscription "šta da?" everywhere.
I wonder what is the meaning of this phrase.
Literally, this means "What? Yes" and makes no sense to me.
"Da" has quite a lot of meanings even in East Slavic languages (probably more than in South Slavic ones, though I never tried to compare). Something like "what if"?
ŠTA DA? is the phrase or a question that in Croatian means something like REALLY?, but that phrase is exceptionally characteristic of the city of Rijeka, and it is often used by locals in conversation.
I'd add that although it does translate as "really?" it actually has variety of meanings based on your tone (it can be ironic, surprised, etc.), hence the wide usage 😅