The part of a man's penis that is hidden from view either under the pubic mound or above the testicles. The inner shaft is not seen but is felt during intercourse.
When a fellow chap thinks with his male genital organ rather than his head
"Look at you John, she took half of your equitable estate, your kinds, and all she gave you is god damn squirrel AIDS, and hepatitis A through Z! Really naive; you should stop thinking with your mind-shaft and start using that bigmelon of yours."