noun. I think it's under the skullbone. And it's connected to the corpus-callosum-bone.
Real citation: "It hurts my thinkbone."
(Jon Stewart doing George W. Bush, Jan. 30, 2006, The Daily Show)
Made-up citation: "According to a recent Harvard study, America's favorite body parts are the think-bone, the boner-bone, and hair. Further research is needed on this problematic subject."