Green’s Dictionary of Slang

1. (UK Und.) a thief.

1653		in Hazlitt Handbk to Popular, Poetical, and Dramatic Literature of G.B. (1867) 320/1: [title] A Total Rout, Or a Brief discovery of a Pack of Knaves and Drabs, intituled Pimps, Panders, Hectors, Trapans, Nappers, Mobs, and Spanners : the description of their qualities is here set down in brief.
c.1698		B.E. Dict. Canting Crew n.p.: Napper c. [...] a Thief.
1712		‘Black Procession’ in Farmer Musa Pedestris (1896) 39: The sixteenth a sheep-napper, whose trade is so deep, / If he’s caught in the corn, he’s marked for a sheep.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1741		Canting Academy, or the Pedlar’s-French Dict. 114: Sheep-stealer Napper of Blaring Cheats.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: napper [a] thief.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1785].
1859		Matsell Vocabulum 58: napper [...] a thief.

2. a false witness.

1984

Partridge DSUE (8th edn) 778/2: C.18.

3. (UK Und.) a cheat.

1674		Head Canting Academy (2nd edn).
c.1698		B.E. Dict. Canting Crew n.p.: Napper c. a Cheat.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1754		Scoundrel’s Dict. 16: A Cheat – Napper.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1859		Matsell Vocabulum 58: napper A cheat.