Green’s Dictionary of Slang

napper n.1

[nap v.1 ]

1. [mid-17C–19C] (UK Und.) a thief.

2. [18C] a false witness.

3. [mid-18C–early 19C] (UK Und.) a cheat.

In phrases

napper of naps (n.)

[late 17C–late 18C] a sheep thief.