Green’s Dictionary of Slang

dirt road n.

also dirt run, old dirt road

[1910s+] the anus.

In phrases

go up the (old) dirt road (v.) (also go down the dirt road, ...up the dirt route) [fig. use of colloq. dirt road, the highway]

[1930s+] to perform anal intercourse.

take it up the dirt road (v.)

[1950s+] to be sodomized.