When you shove your thumb in someone's hole so hard, and with such stamina, that you literally stop them from dying.
It's a good thing Scott was able to Kentucky Plug himself, or else we'd have had to hear all about how his dad did it to him - that that would've been an even weirder boner.
Probing a ladies butthole with ones thumb. - made famous by Gun Meme review by Brandon Herrera, In reference to Scott from Kentucky ballistics, for when he stopped his bleeding, by plugging his severed jugular with his thumb, so he could get to the hospital.
A sexual position, usually done in the backwoods or a gun range, whereas the receiver takes the givers thumb in an orifice, usually while a cameraman films.
He was just sitting down when he got slapped with the Kentucky Plug.... The camera man kept saying, "put your thumb right here!"