This paper examines nineteenth-century ethnonyms for Hungarian in the Hungarian German and Slovak languages in light of the Whorfian hypothesis. Nineteenth-century Hungary witnessed nationalist tensions between Slovaks and Hungarians. These tensions partially took the form of a dispute over the non-coincident meaning of the national ethnonyms for Hungarian: the Hungarian word Magyar conflates ethnic Hungarians with citizens of Hungary, while Slovak distinguishes the Hungarian-speaking Mad'ar from the Uhor, an inhabitant of Hungary who may speak any language. Both Hungarians and Slovaks contested German usage, attempting to influence the prestigious language of inter-ethnic communication. This difference in meaning, however, arose in response to a specific political situation, and the conflict over the meaning of these terms ceased when that situation ended. The history of the Slovak terminology shows that conscious language planning can overcome linguistically imposed obstacles, and that Whorfian arguments themselves play an important role in the thought of political actors.