Green’s Dictionary of Slang

excellently, pleasantly, very well.

1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 161: Slap-up — used for ‘bang-up’.
1840		Courier (Hobart, Tas.) 27 Oct. 3/1: [advert, from UK source] [A] pair of Kicksies, built very slap, with the artful dodge, a Canary and 2 bulls.
1853		‘Cuthbert Bede’ Adventures of Mr Verdant Green (1982) I 126: She’s a girl of what you call mind [...] and she sings regular slap-up, and no mistake!
1888		R. Barnett Police Sergeant C 21 71: They do these things slap up in France.
1938		J.H. Warner ‘A Word List From Southeast Arkansas’ in AS XIII:1 7: slap up, adv. Entirely or fully. ‘He worked slap up till four o’clock.’.