The story
- Proto-Indo-European*ud-s-?
- Proto-Indo-European*h₂u-s-?
- Proto-Germanic*uz-
- Proto-West Germanic*uʀ-
- Proto-Indo-European*sed-
- Proto-Indo-European*sodéyeti
- Proto-Germanic*satjaną
- Proto-West Germanic*sattjan
- Proto-West Germanic*uʀsattjan
- Old High Germanirsezzen
- Middle High Germanersetzen
- Germanersetzenbf.
- GermanErsatz
- English todayersatz
Borrowed from German Ersatz (“replacement, substitute”), a back-formation from ersetzen (“to replace, to substitute”).
Etymology from Wiktionary, CC BY-SA 4.0.