Green’s Dictionary of Slang

1. (US Und.) a police officer or sheriff [? they carry a gun on their hip].

1909

F.H. Tillotson How I Became a Detective 88: A ‘hipper’ is a policeman, but it is a term generally applied to a sheriff’s posse. The ‘main hipper’ is the sheriff himself.

2. (Aus. und.) a hip pocket.

1923

Smith’s Wkly (Sydney) 11 Aug. 15/4: As the bottle passed round she chats us. ‘In his top inside kick most of it, and a couple of funt in his hlpper’.