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the very center of a persons' abs (more common when it's a 4 pack) that is harder than the rest of that abs, and tightens similar to a knuckle. It hurts when hit directly, similar to the pain of playing bloody knuckles.
(out of stupidity) I'm going to break your tummy knuckle!

My tummy knuckle hurts after doing so many crunches.
by Aaron Moe Scott March 30, 2010