Orks are dead killy by themselves. They are huge, hulking creatures, obsessed only with fighting and killing whatever is within their reach - and to a lot of Orks, there are a lot of things to kill within their reach. Whether it be Shootas, Choppas, Rokkit Launchas or even the dreaded Power Klaw, the average Ork Boy has a lot of options availabe to him (it?).
However, to some Orks, that isn't enough. Some can't beat the Orks around them and rise in the ranks, some know that they will be cannon fodder for some Marine's bolter. Their death is pretty much ensured, and although Orks don't generally fear death, they do fear not being able to kill stuff anymore - to an Ork, that is the greatest fear of all.
Long had the Oddboys been discussing this matter, trying to solve this grave problem. Experiments ensued, all without success. In the end, it all seemed futile, until one enterprising Mek had a brilliant idea, inspired by the bludy humies no less. He had been fighting some Space Marines like usual, looting guns and exploding stuff. Suddenly, he saw the most beautiful machine he had ever seen. Boxy, heavily armoured and loud as hell, the machine tore through the Ork Boyz at an alarming rate, destroying everything and anyone, screaming with a metallic voice "EVEN IN DEATH I STILL SERVE". The Machine was a Dreadnought, a Walker with a dying Space Marine within its shell.
The Mek thought he had seen a vision. He went home and began building a mechanical walker, armed with rockets, ripping blades, guns and swords, with a cockpit with just enough room for an Ork of dimunitive size. Soon he found an Ork of relativly small size, an ork who was probably dead anyway, and had the Painboy hardwire him, whereafter he intered the Ork into the machine. The Ork was filled with rage and passion, rushing out to show the clan his new, mechanical body by showing their internal organs to them.
The Mek dubbed the machine, "Deff Dread", because this machine cured the only thing that Orks feared -
the dread of dying before having a proper fight. Not really. They called it "Dred" as in dreadnought, and since it was so ded-killy they threw "Deff" in there.