Green’s Dictionary of Slang

nap n.1

[lit. and fig. uses of nap v.1 ]

1. [late 17C–early 18C] a dose of venereal disease.

2. [late 17C–early 18C] an instance of cheating while playing dice.

3. [mid–late 19C] (orig. theatre) a blow or hit, esp. a pretend hit.

In phrases

nap (it) (v.)

[late 17C–early 19C] to catch venereal disease.