Green’s Dictionary of Slang

(US) a police officer.

1890		Eve. World (NY) 4 Aug. n.p.: Charged the Wrong Clubber. Policeman Mahoney Instead of Sullican will Stand Trial.
1899		Sun (NY) 20 Aug. 7/4: Alleged Clubber held for Trial. Policeman Patrick Carmody [...] on charge of feloniously assaulting Charles A. Murphy.
1904		Eve. World (NY) 22 Dec. 12/1: Clubber Held for Assault. Policeman Frank McLaughlin Who without provocation Brutally Attacked Reporter Robbins Faces Criminal Trial.
1915		S. Ford Shorty McCabe on the Job 61: Slug the clubbers! [...] Knock their blocks off! Go to it, old man!