I suppose it would actually mean. What is that you are drinking? (Surprise and shock) Could it be milk? Or its like saying, I can't believe it are you drinking milk?
Meaning and exact word for word translation don't always correlate.
I have trouble in transliterating.. When I see the azbuka sentence and I have the "chto eto ty pesh moloko" it looks identical to me... Is it the word "pesh / pyosh" - should I have been more phonetic here? Or is it elsewhere?