When you're getting hot and heavy with a girl and she starts her period so sticking your thumb up her vagina to stop the bleeding.
I was starting to have sex with a girl off tinder when she suddenly started her period, I gave the the Kentucky thumb and let her give me head instead.
The Kentucky Thumb is a sex position in which you stick your thumb in your partner's ass and/or vagina while she's cumming. God may disapprove, but Mat will be happy.
*Wife comes home.* Wife:"Can we try something new?" Husband:"Howabout the kentucky thumb?" Wife:"What's that?" Husband:"You'll see."