Green’s Dictionary of Slang

first-rate, excellent.

1844		New Sprees of London 3: I’ll introduce you to [...] the Lushing, Chanting, and Night-cribs [...] where you may doss lush, or feed, from the slap club houses of St. James' to the shysiest of the Cadgers’ Kens in the back slums.
1848		‘Epistle from Joe Muggins’s Dog’ in Era (London) 9 Apr. 3/3: [I] submitted my morning toylet to the careful superhintendince of ‘Old Joe,’ who dressed out my tail, iled the jints, pointed my ears, and made me look astonishing slap.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor II 107/1: People’s got proud now, I fancy that’s one thing, and must have everything slap.
c.1865		‘Mary Newall’ in C. Hindley Curiosities of Street Lit. (1871) 140: Drest up in slap togs.