An Etymological Dictionary of the German Language/Annotated/Reuter
Reuter, masculine, ‘trooper,’ first occurs in early Modern High German, formed from Dutch ruiter, ‘trooper,’ which has nothing to do with reiten, ‘to ride.’ The word is based rather on Middle Latin ruptarii (for ruptuarii), rutarii (ex Gallica pronuntiatione); thus were “dicti quidam praedones sub XI. saeculum ex rusticis collecti ac conflati qui provincias populabantur et interdum militiae principum sese addicebant”: “these people were often on horseback.” Thus Dutch ruiter could easily acquire the meaning ‘horseman’; compare Dutch ruiten, ‘to plunder.’ See Rotte.