Noun: A sex act. One partner uses their non dominant thumb by inserting their thumb into the anus of the other partner while giving oral sex.
The dominant partner performs a Kentucky Thumb applies pressure to the anal interior with the thumb and places the remaining 4 fingers one of the posterior cheeks, gripping it like a Basketball. All while preforming oral sex on the recipient. This position can be done in either the 69 position or in the standing position.