Where does “-er” come from?

-er (Old Frisian) comes from Proto-West Germanic iʀ, from Proto-Germanic jīz, from Proto-Indo-European *yúHs — you (plural).

Ancestry of “-er”, step by step

StepLanguageWordMeaning
1Proto-West Germaniciʀhe, it
2Proto-GermanicjīzAlternative form of *jūz
3Proto-Indo-European*yúHsyou (plural)
Every word from Proto-Indo-European *yúHs →Every word from Proto-Germanic jīz →Every word from Proto-West Germanic iʀ →