A simple electrical cord that has prongs on both ends. Thus, if you plug one end in to a live outlet, the exposed prongs are energized and dangerous.
Such a device is actually useful, for example, to "inject" power into a dead circuit from a generator. However, due to the inherent safety issues, you generally can't find them for purchase, nor would a licensed electrician ever provide you with one. Thus, do-it-yourself handymen are left to splice together their own version of this device (probably making them unsafer).
When the hurricane hit and the power went out, I fired up the generator and jacked it in to the outlet in my garage with a suicide cable; at least I was able to run a few lights and my refrigerator.