This term for an FBI agent dates back to before the 1920s
[DOC] [TLE] Prohibition: Roots of Prohibition | PBS [PAR] Just two weeks earlier, on the last New Year's Eve before Prohibition, frantic 
celebrations had ... WWI Poster "Will You Back Me or Back Booze", Ohio 
Historical Society ... At 12:01 A.M. on January 17, 1920, the amendment went into 
effect and ... Would you be an FBI agent, bootlegger, flapper, teetotaler or 
mobster?