Green’s Dictionary of Slang

pee-man n.

also P’man
[abbr.]

(UK Und.) a policeman in civilian clothes.

[US](con. 1910–20s) D. Mackenzie Hell’s Kitchen 119: Pee-man [...] policeman in civilian clothes, called in the Force Capman, to distinguish from the C.I.D.
[UK]Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 8: Pee-man or P’man: Policeman.