"The Old High German chuphar, Middle High German and modern German kupfer correspond to West Germanic type *kuppar < popular Latin cuprum. The Low German forms point according to Pogatscher, to a variant Latin form *coprum (whence also Old French coevre )" (OED).
Lübben, August (1888). Mittelniederdeutsches Handwörterbuch. Norden: Soltau
Robert Farren, PIE culture words collection, 2017
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