The word 'heath' is defined as barren land, wasteland, uncultivated land. It refers to land found in Northern Europe. 'Heathen' is the name for those who lived on the heath - the Heath-men. These were the Vikings who, unable to survive from cultivating this barren terrain, crossed the sea to pillage the cultivated lands of others.
Today the word 'Heathen' is a derogatory term used by Christians to describe their enemies, opponents etc., and though it's been a thousand years since the Vikings terrorized the Christian nations of Europe, their depradations and the fear they aroused must have been extraordinary to have left such a mark on the psyche of Christianity that the term 'Heathen' exists to this day to denote all that is the antithesis of Christian thought and belief. Jews for example call their enemies anti-semites, not 'Romans' despite the fact that Rome destroyed Israel, burned the temple, and caused a diaspora of almost 2000 years.
A book called 'The Vikings' by Robert Ferguson gives some insight into what the Vikings were all about. Like this example of the inhumanity of the Vikings:
In the year 1014 in a sermon, Wulfstan, Archbishop of York, abominated the shameful Viking practice of men banding together to buy a female slave to use for their sexual gratification before returning her to the auction block to sell her on down the line.
Here's his vivid description of captives being herded, probably to ships, to be transported to a life of slavery - "Often two seamen, or maybe three, drive the droves of Christian men from sea to sea, out through this people, huddled together, as a public shame to us all" - a vivid picture of a demoralized English populace. Two or three Viking guards - all it takes to control a crowd of Anglo-Saxon Christians; like SS guards in Auschwitz.
Slave trading was by far the greatest source of income for the vikings. The source of the slaves was the British Isles in the west and the lands of the Slavs in the east. But such was the volume of human trafficking in captives taken from among the Slavs that the term "Slav" became, via the mediaeval Latin sclavus, our word for all humans held in bondage - "slave".