Green’s Dictionary of Slang

second-rate, worthless.

1962		R. Cook Crust on its Uppers 28: They were just slag punters and couldn’t pay.
1983		A. Payne ‘Get Daley!’ Minder [TV script] 52: Now what about Harrison’s slag brother.
1999		Guardian Guide 6–12 Nov. 6: Do you feel lucky? Well do you, you slag ponce?