Green’s Dictionary of Slang

passenger n.

1. [mid-19C+] one who, while nominally one of a group, team, crew etc, takes no active or useful part in the general efforts.

2. [late 19C–1930s] (US) a passenger train; thus passenger stiff, a tramp who rides passenger or fast freight trains.

3. [2000s] (US prison) a friend [i.e. one who is in the car n. (1)].