In Long: A normal man, exposed to radiation by accident one day at work at an army base. His body was torn apart, literally. But he was able to rebuild himself piece by piece. He is America's greatest weapon because he has a high level of radiation surrounding him, and can disintegrate his enemies on contact. He wore clothes twice in the movie "Watchmen". All the other times, he was naked, and nobody seemed to mind, or even notice. Just see the movie, it explains alot of stuff.
A dildo or other phalic shaped object is also acceptable to use, but a real penis is preferred.
Derived from the Watchmen comic/film character who was blue and naked.
"I'm bored" "Bro, fuckin Dr. Manhattan it up"
Dr. Manhattan'ing makes my cock look great!
(like Dr. Manhattan clones himself and performs sexually with Laurie in "Watchmen")
This can also apply, though less often, when two twin females perform the same act. The common term for this is "AWESOME."
Also as a verb:
Jim and Bill completely Dr. Manhattaned Jim's girlfriend last weekend, and she LOVED it!