Green’s Dictionary of Slang

a form of ‘what’ used when questioning the previous speaker’s self-description, e.g. ‘I’m a butcher.’ – ‘A what-er?’.

1909		J. Ware Passing Eng. of the Victorian Era.
1909		J. Ware Passing Eng. of the Victorian Era 262/2: Whaty? A (Peoples’, 18 cent.). An enquiry made when any strange statement is made: e.g., ‘He’s a jimjack’ – ‘A whaty ?’- ‘A jimjack.’.