Green’s Dictionary of Slang

suddenly, immediately; violently.

1678		Dryden Kind Keeper III i: Down I put the Notes slap-dash.
1693		Congreve Old Bachelor IV iv: Now am I slap dash down in the mouth, and have not a word to say!
1715		‘Nickydemus Ninnyhammer’ Homer in a nut-shell 49: He quickly springs from his Gallash / To fall upon the Pimp slap-dash.
1730		J. Gay Wife of Bath (rev. edn) V i: In the instant she consents, you shall nick them slab-dash [sic] with the ceremony.
1739		Roger Bull Grobianus 145: None in the Bason cares to plunge Slap-dash.
1745		A. Hamilton Tuesday Club Bk III in Micklus (1995) 53: Mr Jole proceeded gradually in his Schemes, and slap dash, there followed a whole troop of frecassees, ragous, hashes, soups.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
c.1791		Kilmainham Minit in Walsh Ireland Sixty Years Ago (1885) 88: Slap dash tro de Poddle we lark it.
1797		Bridges Burlesque Homer (4th edn) I 202: Which made him in a mighty passion / The poor Ulysses fell slap dash on.
1800		Sporting Mag. Oct. XVII 23/1: Riding all day slap dash through the public streets.
1811		J. Poole Hamlet Travestie III iii: And sous’d her over head and heels, Slap-dash into the water.
c.1815		J.K. Paulding Bucktails (1847) V ii: A little space of health, and then slap-dash comes the quaver again.
1822		‘A. Burton’ My Cousin in the Army 133: See, here’s the gash Through which this blade went—right slapdash.
1824		‘Geoffrey Crayon’ Tales of A Traveller (1850) 219: I talked, and rattled, and said a thousand silly things, slap-dash.
1834		T. Hood ‘Tylney Hall’ Works (1862) III 240: I always broke slapdash through his guard.
1847		J.M. Field Drama in Pokerville 146: The doors opened with a slap-dash!
1859, 1860		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1895		Crowe Aus. Sl. Dict. 75: Slapdash, quickly, at a quick pace.
1905		L. Pound ‘Dialect Speech in Nebraska’ in DN III:i 65: slab-dab, slap-dash, intensive adv. [...] ‘It fell slap-dash into the water.’.
1912		E. Pugh City Of The World 272: You watch your chance. And when it comes you take up the wrong bag by mistake and dander off with it. That’s one way – the brisk and slap-dash way.