Heathen as far as I know means people of the heath, which is the shrubland habitat outside of villages with poor acidic soil and low woody brush. Basically the people who were kicked out of the good fertile farmland and forced to live outside of society.

Also if it’s high and hilly heath becomes moors, so your condescending teenage fling very smugly gave you the exact opposite of a correct definition because heaths are always lowlands.