An Etymological Dictionary of the German Language/Annotated/Rüpel
Rüpel, masculine, ‘coarse fellow, lubber,’ properly a Bavarian abbreviation of Ruprecht (hence Rüpel and Rüppel as surnames); for a similar use of proper names compare Metze. Probably the meaning of Rüpel was occasioned by Knecht Ruprecht, ‘Knight Robert’ (bugbear in nursery tales), in whose character maskers disported themselves in a rude and coarse manner.