Green’s Dictionary of Slang

rural n.

(US) a country policeman.

[US]Whip & Satirist of NY & Brooklyn (NY) 10 Sept. n.p.: At the ground they were disturbed by the ‘rurals’ [...] on reaching Shere Mere another policeman was on the look-out .

In phrases

do a rural (v.)

1. to have sex in the open-air.

[US]Maledicta IV:2 (Winter) 195: A […] stargazer, which can also be a hedge-whore – that is one who goes stargazing on her back and receives an Anglo-Indian back. In other words, she has done a rural.

2. to urinate or defecate in the open.

[UK]Partridge DSUE (8th edn) 317/2: C.20.