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A hand game starting off at the thumb, counting off fingers until you get to the pinky. At "sound," you bend back the pinky as hard as you can and the sound that your friend makes is the sound they will make during sex. Popular with the teen crowd, can sometimes result in broken pinkies.
"Hey, can I see your hand?"
"Sure dude, what for?"
"This" thumb "is" pointer "your" middle "sex" ring "sound" pinky

- "This is your sex sound" nuff said.