Green’s Dictionary of Slang

napper n.1

[nap v.1 ]

1. (UK Und.) a thief.

[UK]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.
[UK]B.E. Dict. Canting Crew n.p.: Napper c. [...] a Thief.
[UK]‘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.
[UK]New Canting Dict. [as cit. c.1698].
[UK]Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
[UK]Canting Academy, or the Pedlar’s-French Dict. 114: Sheep-stealer Napper of Blaring Cheats.
[UK]Grose Classical Dict. of the Vulgar Tongue n.p.: napper [a] thief.
[UK]Lex. Balatronicum [as cit. 1785].
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1785].
[US]Matsell Vocabulum 58: napper [...] a thief.

2. a false witness.

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

3. (UK Und.) a cheat.

[UK]Head Canting Academy (2nd edn).
[UK]B.E. Dict. Canting Crew n.p.: Napper c. a Cheat.
[UK]New Canting Dict. [as cit. c.1698].
[UK]Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
[UK]Scoundrel’s Dict. 16: A Cheat – Napper.
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Lex. Balatronicum.
[US]Matsell Vocabulum 58: napper A cheat.

In phrases

napper of naps (n.)

a sheep thief.

[UK]Head Canting Academy (2nd edn) 176: Napper of Naps A Sheep-stealer.
[UK]B.E. Dict. Canting Crew n.p.: Napper c. [...] a Thief [...] Napper of Napps A Sheep-stealer.
[UK]Scoundrel’s Dict. 19: A Sheep-stealer – Napper of Nays [sic].
[UK]Grose Classical Dict. of the Vulgar Tongue n.p.: napper of naps, a sheep stealer.
[UK]H.T. Potter New Dict. Cant (1795) n.p.: napper of napps a sheep stealer.
[UK]G. Andrewes Dict. Sl. and Cant n.p.: Napper or [sic] naps a sheep’s-stealer.
[UK]G. Kent Modern Flash Dict. 23: [as cit. 1809].
[UK]Flash Dict. in Sinks of London Laid Open [as cit. 1835].