Green’s Dictionary of Slang

peek n.

[SE peek]

1. [1940s] (US) a peephole, e.g. in a brothel.

2. [1940s–50s] (UK prison) an observation cell, into which prisoners are placed if, for instance, they have smashed up their cells or shown other signs of instability.

In compounds

peek freak (n.) [freak n.1 (7)]

[1960s+] a homosexual voyeur who watches two other men during sex.

peek show (n.)

[2000s] a live sex show.