Therefore, let me be counted as the fifth wheel.

This is the oldest attestation of the figurative metaphor of the “fifth wheel on the wagon” in the German language, a phrase already documented in Latin as early as the 11th century (Germania 18,31 5): "Quem fastidimus, quinta est nobis rota plaustri". ("Wer uns lästig ist, der ist uns das fünfte Rad am Wagen"). See Lutz Röhrich, Lexikon der sprichwörtlichen Redensarten, entry “Rad”, p. 4896 (oldphras). Herbort mentions that the story of the conquest of Troy already existed in Greek, Latin, and "welsch" ('Romance', i.e. French). Since he was to be the fourth to treat the material (in German), another, however, got ahead of him (namely Heinrich von Veldeke with his Eneasroman). Therefore, he accepts being regarded as the fifth wheel on the wagon. (Original: "So zele man mich zv dē fūftē [= dem funften] rade". — Handschrift Cod. Pal. germ. 368. fol. 1v (digi.ub.uni); Herbort von Fritslâr's Liet von Troye, ed. Ge[org] Karl Frommann (Quedlinburg und Leipzig, 1837) p. 2, l. 83 (Google Books)