When someone encircles your butthole with their finger (preferrably pinky) while orally stimulating you, gradually easing the finger inside of you until they are knuckle deep in your pooper. Surprisingly sensual and recommended for uninhibited and particularly dirt nasty couples. Named after the equally surprisingly erotic musical artist Regina Spektor.
Me: Dude, Wayne gave me a Regina Sphinkter last night unexpectedly and who knew I was such a filthy, filthy whore!
Them: Um, everyone?
Me: That's beside the point! Reg Sphinks rule!!!!