Pain or stiffness in the thumbs occuring most often after a long bout of playing a game on the SEGA console.
Can also be applied to other consoles.

When you eat dirt for the first time and leave your family to build a shack in the woods

Sega Thumbs are human thumbs that bend backwards, under their own power, so far as to reach a Reverse Right-Angle. Almost all of our thumbs bend forward 90 degrees. The Left or Right Reverse Right-Angle is a feat most often achieved by Sega fans who were around when the games we are playing on our cell phones were the games that were proudly displayed on our brand new WOOD TVs!
Bonus1:Using 1 or more Sega Thumbs speeds up Text Messaging.
Bonus2:Hitch-Hiking with Sega Thumb is a guaranteed ride.
Bonus3:Sega Thumbs give great Massages.

A disease caused by playing Sonic Spinball on the Sega MegaDrive for 57 hours straight. Known to lead to cripplingly premature arthritis.