I was wondering if a native speaker would use "What's up" instead of " What's that" when someone didn't hear what others said and simply asks what it is. Because I think I heard that usage but I'm not sure if it means the same thing.
It always used to mean “What’s the matter?”. It’s now come into use as a type of greeting, for some people. But never, in my experience, is it used to mean I didn’t hear what you said!
Thank you. I must have misheard it then. Last question. Is there anything someone would say in that situation and begin with "What's ....". Other than "what's that" obviously.