Green’s Dictionary of Slang

1. the food available at a slap-bang(-shop) n.

1797		‘T.B. Junr.’ Pettyfogger Dramatized II i: Come, we’ll get a plate of slap-bang, and a pot and a pipe at the Colt’s Foot [Ibid.] 109: Slap bang. A-la-mode Beef.
1838		‘Paul Pry’ Oddities of London Life I 160: [T]he father only laughed, and swore that ‘no slap-bang seller should put a finger on his progeny’.
1848		Flash Dict. in Sinks of London Laid Open.

2. a stage-coach.

1797		M. Robinson Walsingham IV 9: I invented the slap-bang coaches, and sported the tandem.
1827		‘Patroclus’ q. in Sporting mag. in Malet Annals of the Road 92: An Oxford slap-bang, loaded in London.

3. a form of alcoholic drink [used by Benjamin Disraeli in Sybil (1845) as ‘the Mowbray slap-bang’, but otherwise unspecified. Presumably it was tossed off in a single gulp, guaranteed to ‘hit the spot’ or, like the tequila slammers of the present day, the glass was knocked against the table before taking a drink].

1821		‘A Real Paddy’ Real Life in Ireland 48: By the piper that played before Moses! I’ll have an action of assault and battery against him if Macanally’s in Dublin, or slap-bang to be bought in Stafford Street.
1850		Bell’s Life in Sydney 9 Feb. 3/4: The Cafe de Slap Bang and Sheeps Trotterums.