Prinzip is German for “principle” and a Reiter is a “rider” (of horses). Hitch the two words together, and you have a person who acts on principle, as a matter of principle, in the most inflexible, even bone-headed way no matter what the consequences are. I’ve found, both in Germany and in the US, Prinzipienreiter (singular and plural are the same for both nominative and accusative!) especially among the ranks of low-level officials who get their authority not from their expertise or charisma, but solely through their position, and are willing to use what little power they have to the max by following procedures to the tee.
Here’s an incident during this year’s mardi-gras in Cologne that shows beautifully Prinzipienreiter at work (I’ll summarize the incident in my first comment for people who don’t speak German).
Note on pronunciation: Prin·TSEE·pee·en·RYE·ter.
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