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."
To use the thumb on a woman's clit long enough to expose the hard clit from beneath it's sheath.
"Dude, how did it go last night? Did you get her off?"
"Yeah, I had to thumb the urchin for a while, but eventually that little bastard came out to play"