대체 뭐야?

도대체 do dae che

you can say 이건 뭐야? which means what is this? but the expression is little soft.. so insert an expletive word in front. <expletive> 이건 뭐야?! <expletive> ikun mwoya?! ee-kohn moh-ya You would have to only say this to someone you know quite well though. I'm sure someone will post if there is a more specific expression.