Green’s Dictionary of Slang

napper n.2

1. [late 17C–19C] a hat [? nab n.1 (3)].

2. [early 18C+] (also knapper, napper box, napper tandy, nopper) the head.

3. [early 19C] a nose.

4. [late 19C–1920s] a face.

5. [late 19C–1920s] the mouth.

In compounds

napper-case (n.) [mid-19C]

1. a wig.

2. the head.

In phrases

do on one’s napper (v.)

[late 19C] to achieve something easily.

go off one’s napper (v.)

[late 19C] to go mad.

have a napper (v.)

to be intelligent, to ‘have a head on one’s shoulders’.