Putting your whole thumb into a buthole during sex. Can be performed by a partner or solo.
The saying originates from an accident that occured when the popular gun-youtuber Scott DeShields or "Kentucky
Ballistics" was shooting his 50.Cal
sniper rifle. The rifle exploded and sent shrapnel in his neck and caused severe blood loss. To save his life Scott put his thumb into the whole where the shrapnel had
penetrated his skin and thus making it to the hospital and surviving.
After the accident the gun community exploded with memes about his thumb and ultimatley the "Kentucky Thumb" was created.