To make someone feel less than what they are. To take away someone's individuality, compassion, or civility. Slaves were dehumanized. They were one race that were all treated the same. They were treated like they had no life or even humans.
You dehumanize a man as much by returning him to nature - by making him one with rocks, vegetation, and animals - as by turning him into a machine. Both the natural and the mechanical are the opposite of that which is uniquely human. Nature is a self-made machine, more perfectly automated than any automated machine. To create something in the image of nature is to create a machine, and it was by learning the inner working of nature that man became a builder of machines. It is also obvious that when man domesticated animals and plants he acquired self-made machines for the production of food, power, and beauty.
Eric Hoffer (1902 - 1983)