Green’s Dictionary of Slang

top whack adv.

at the most.

1958		F. Norman Bang To Rights 154: A magestrate can only give you a stretch top wack.
1965		Sun. Mirror (London) 5 Sept. 8/6: [We] skipped off by train to Scotland. £15 a week, top whack.
1979		F. Norman Too Many Crooks Spoil the Caper 99: Your average Mickey will spark you out for no more than twelve hours top whack.
1989		in G. Tremlett Little Legs 42: I was allowed five minutes with them, top whack.
1997		P. Theroux Kowloon Tong 53: ‘He’s offering us seven-plus,’ Betty said. ‘Top whack. A million quid.’.
2000		J.J. Connolly Layer Cake 168: It’s all taken about three or four seconds, top whack.