Green’s Dictionary of Slang

first-rate, excellent; also as an intensifier.

c.1776		E. Bangs ‘The Yankee’s Return From Camp’ in Silber Songs of Independence (1973) 78: He got him on his meeting-clothes, / Upon a slapping stallion.
1824		J. Wight Mornings in Bow St. 77: The waiter instantly brought him what they call ‘a plate’ of roast beef — several good jolly slapping slices, swimming in twelve-water gravy.
1835		D. Crockett Col. Crockett’s Tour to North and Down East 224: A slappin big angel has him under the armpits.
1838		R.S. Surtees Jorrocks Jaunts (1874) 95: A fine slapping four-year-old.
1843		Bellew Memoirs of a Griffin I 52: [I] was conveyed at a slapping pace to my host’s garen residence.
1865		H.L. Williams Joaquin 20: It’s a slapping bit of horseflesh between your legs, Bill!
1871		Waterford Standard 29 Mar. 4/3: [A] good fox, which led them at a slapping pace towards Durrow.
1897		Truth (London) 10 June 35/1: ‘Slappin’ old gal, isn’t she [...] she’s my missus’.