Green’s Dictionary of Slang

on adv.²

no.

1859, 1860		Hotten Dict. of Modern Sl. etc.
1873		Sl. Dict.
1890		Northampton Mercury 12 Apr. 12/4: ‘Shif,’ fish; ‘torrac,’ carrot; ‘yad,’ day; ‘kool,’ look; ‘sey,’ yes; ‘on’, no; ‘pac,’ cap; ‘reeb,’ beer, ‘yob,’ boy; are all perfect specimens [i.e. of backslang].
1953		J. Franklyn Cockney 298: Had the manager replied Say (Yes) instead of On (No) the customer would have gone away happy.