Why is a Deadman called so? Where does the name come from?
imsodin's explanation (it's a heavy object you bury in the ground, like a dead body) seems very plausible. Just to add some history to it, here's the earliest use of the term listed in the Oxford English Dictionary:
The next recorded usage is:
So the term has existed in agricultural and nautical contexts for some time (and when you're burying something in earth rather than snow, the connection with burying a body is a little more obvious).
The DMM device you linked to is as far as I know the only product that has this name officially, but a dead man anchor is a general term for what is in mountaineering also called a t-slot anchor. In that case you simply dig a trench at right angle to the direction of pull, bury an object and attach a rope that holds the weight. I presume that as the DMM device is applied in similar circumstances (it is more dependent on very compact snow but faster) it has the same name.
There is an explanation for the name itself, but beware of obvious morbidity: