Green’s Dictionary of Slang

peeper n.

[SE peep, to look at]

1. [mid-16C+] an eye, usu. in pl.

2. [late 17C–19C] a looking-glass, a mirror.

3. [early 18C] glass, e.g. a window.

4. [late 18C–mid-19C] a telescope, a spy-glass.

5. [early 19C] in pl., spectacles.

6. [late 19C+] a police officer; a security officer, e.g. in a hotel.

7. [20C+] (US) in pl., sunglasses.

8. [1940s] a private investigator, with implications of voyeurism.

9. [1940s+] a Peeping Tom, a voyeur.

In phrases

painted peeper (n.)

[late 18C–1940s] a black eye; in pl., a pair of black eyes.

peel one’s peepers (v.) (also peel one’s peeps)

[late 19C-1910s] to keep a lookout; to look wide-eyed.

peeper in mourning (n.)

[19C+] a black eye; usu. in pl.