dēr (Old Frisian) comes from Old Saxon dior, from Proto-West Germanic deuʀ, from Proto-Germanic deuzą, from Proto-Indo-European dʰewsóm, from Proto-Indo-European dʰéws, from Proto-Indo-European dʰwes- — to breathe; breath; spirit, soul > creature.
Ancestry of “dēr”, step by step
dēr traces back through two separate lines of ancestry.