When I talk to somebody, and I miss a certain part(word) of the sentence they said, I have a habit of replacing the unheard part with what and ask the same sentence like this:
Speaker: Can you please pass the texta(missed)?
Me: Can you pass what?
Speaker: Why don't you drink coffee(missed) in the morning?
Me: Why don't you drink what in the morning(what-in-the-morning is spoken with a special intonation).
Speaker: What is the time(missed)?
Me: What is what?
The speaker I was talking to was an English guy, and he said you can't say what's what.
What is grammatically wrong with what's what as a question and why can I replace the unheard word with what in other sentences?