Originally coined by Invader Zim's malfunctioning SIR Unit, GIR, in the now classic "Doom Song", it has been adopted by humans aware of our own impending destruction. Whether our destruction comes by the Mighty Irken Invaders, our pissed off planet or by our own myopic stupidity, only time will tell.
It is the mental equivalent of Wile E. Coyote's umbrella
Meteor, Volcano, disease, pollution, all too much? Just say 'doomity-doom!' and embrace the DOOM!