Green’s Dictionary of Slang

anything put together quickly but not well.

1890		‘’Arry on the Sincerest Form of Flattery’ in Punch 20 Sept. 144/2: These jossers [...] / Who himitate ’Arry’s few letters with weekly slapdabs of bow-wow.
1927		Nottingham Eve. Post 2 Mar. 3/2: You know Old Slap-dab don’t you? He’s the chap who climbs up a ladder and sloshes about [...] and lets trickles of whitewash run down his collar.