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.
[1960s+] a homosexual voyeur who watches two other men during sex.
[2000s] a live sex show.