Green’s Dictionary of Slang

dirt road n.

also dirt run, old dirt road

the anus.

[US]H.N. Cary Sl. of Venery IV (t/s) n.p.: (human body) The Anus [...] dirt road.
[US]Monteleone Criminal Sl. (rev. edn).
[US]Trimble 5000 Adult Sex Words and Phrases.
[US](con. 1930s–40s) B. Rodgers Queens’ Vernacular 108: Hobo slang (kwn ’30s & ’40s) The ass itself became the dirt run, old dirt road and old rip. These words seem to indicate that pederasty was more important than fellation among the homosexual hoboes.

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]

to perform anal intercourse.

[US] ‘Adventures of a Fuller Brush Man: “The Amorous Mrs. Twirp”’ [comic strip] in B. Adelman Tijuana Bibles (1997) 47: ‘Up the Dirt Route’ Very popular with wolves, Greeks and other perverts.
[US]D. Maurer ‘Prostitutes and Criminal Argots’ in Lang. Und. (1981) 116/2: To go up the dirt road. To practice pederasty.
[US]G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
[US]Guild Dict. Homosexual Terms 12: dirt road, to go up the (old) (v.): To pedicate.
[US]R.A. Wilson Playboy’s Book of Forbidden Words.
[US]R.O. Scott Gay Sl. Dict. [Internet] anal intercourse: [...] Syn: goin’ down the dirt road.
[US]B. Keane ‘Dysfunctional Family Circus’ [Internet] I told you before we came up here, if Dad has too much to drink and wants to go ‘Up the Old Dirt Road’, it isn’t because he has to take a piss!
Sexionary at [Internet] Go up the old dirt road – anal sex.
take it up the dirt road (v.)

to be sodomized.

Movie Films Fest ‘Legacy’ Rev. on Excalibur Films [Internet] Wide, hard cocks aren’t a problem for either of these anal sluts as both of them take it up the dirt road with slutty glee.