Green’s Dictionary of Slang

rimming n.

[rim v. (2)]

(orig. US) anilingus.

[US]Maledicta III:2 218: Australian = anilinctus practitioner (rimming from Down Under).
[US]K. Vacha Quiet Fire 45: Rimming wasn’t a big thing in the United States after the war.
[US]Backlash29 ‘swimslut’ on Nifty Erotic Stories Archive 🌐 Chris was a serious rimmer, and within seconds he had squished his entire tongue into Ben’s clean, tasty hole and was swabbing it around. Savoring the heavenly rimming, Ben relaxed.