It seems to be an American word used when something is so obvious that the person asking sounds stupid, but it's not really used nowadays. You might want to take a look at this thread: https://forum.wordreference.com/threads/doy.1871852/ Hope it helps!

I think it's the wrong spelling.. is it 'd'oh!'? But it's just a reaction word like 'wow', 'ah' or something. That's all

Doy = No kidding! (old way to say)