Green’s Dictionary of Slang

(Anglo-Irish) a pickpocket.

1821		‘A Real Paddy’ Real Life in Ireland 15: As the Mint in London was formerly [...] considered a privileged place for whacks and schedams (pickpockets and rebels).
1837		‘The World’s an Assortment of Shoes’ in Sam Weller’s Favorite Song Book 4: A minister’s a cobbling wight, / Who thinks the poor nothing but ‘whacks.’.