A large, long-barreled, high-power handgun, preferably a revolver. From the comic book Preacher where the villain, Starr gets his junk bitten off by a trained Rottweiler and compensates with an obsession with large handguns. He stands in front of the mirror, holding his gun at groin level, saying "doom cock".
That .500 S&W is a fucking doom cock.
Here's where Dirty Harry pulls his doom cock.
Insertion of penis into each ear hole. One penis per ear hole equaling a total of two penises inserted into the head at once.
At Western Illinois University, a young woman was doom cocked in the elevator by Mark and Max.