In 1983, Lee was paying a visit to the Marvel offices when they asked a weird favor of him. They were in the middle of preparing a joke-y issue full of stuff like office pranks, staff members doing superhero things, you know the drill -- would he, they wondered, be into doing a nude centerfold? His clothes were probably in a pile on the floor before they finished asking.
When Lee returned home, however, and remembered what he'd done, he demanded that the centerfold be canned. As a compromise, they did the next best thing and superimposed a massive Hulk over his body. That is not a euphemism.
As for the pube-covered comic he used to cover himself, we're assuming it sold for several billion dollars.