When you're nailing your sister from behind and you slip in the back "accidentally" so you shove your thumb in her asshole so blood doesn't get on your bed.
Had an accidental intrusion with sis last night and had to give her the Kentucky thumb to keep the sheets clean
When you're girl is on her period and she lets you put your eggplant in her truffle tunnel on top of a folding table, but you don't want any blood on your ham candle so you "stick a thumb in it."
Got to ride the mud slide last night, but it was that time of the month so I gave my girl the "Kentucky thumb".