Used to describe the feminine or "girlie" side of a masculine or straight-acting gay man which is typically hidden (often requiring great effort) but that will occasionally be revealed in an unguarded moment. Exposing this feminine side, even momentarily, results in diminished masculinity and weakness.
Dennis: "I used to think Tom was the roughest, toughest bear I had ever met, until he squealed like a 5 year old girl when he saw that mouse!"
Peter: "Yeah dude, he showed us some serious undernelly a minute ago."
Dennis: "After seeing his undernelly I'll never look at him the same way again."