( informal )
1.
used when you have not heard or have not understood sth :
What? I can't hear you.
2.
used to show that you have heard sb and to ask what they want :
'Mummy!' 'What?' 'I'm thirsty.'
3.
used to express surprise or anger :
'It will cost $500.' 'What?'
'I asked her to marry me.' ' You what? '