Green’s Dictionary of Slang

1. a bailiff [SE tap on the shoulder].

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. a blow, a punch.

1828

‘A Flat Enlightened’ Life in the West II 89: Paddy [...] gave the hellite a smart ttapper n. (2)1828napper upon his smeller, which laid the ‘sharp’ flat upon the play table.

3. (US Und.) a policeman [tap v.² (3)].

1859

Matsell Vocabulum.

4. a cadger, a beggar.

1908		Sporting Times 7 Mar. 1/4: ‘No, not a sixpence!’ said Master, driving off a tapper who waylaid him.
1936		W.A. Gape Half a Million Tramps 41: She got as many ‘swag sellers’ and ‘tappers’ as she could to take beds.
1939		J. Worby Spiv’s Progress 32: I didn’t have time to light a cigarette before I was accosted by a tapper who said, ‘Got a fag, mate?’ .
1947		C.S. Montanye ‘Frozen Stiff’ in Popular Detective Mar. 🌐 She had married Tommy the Tapper a short time after she had cached the legacy.
1957		C. MacInnes City of Spades (1964) 73: ‘Come, little white friend,’ said the tapper, in a soft, gentle, stupid, persistent voice. ‘Give me some sustenance.’.
1981		(con. c.1905) A. Harding in Samuel East End Und. 86: His brother Ginger was a character from the Nichol. He was a ‘tapper.’.
1989		J. Morton Lowspeak 137: Tapper – a beggar.

5. (N.Z. prison) an informer.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 185/1: tapper n. an inmate informer, a nark.