An Etymological Dictionary of the German Language/Annotated/Koppel
Koppel, feminine and neuter, ‘leash,’ from Middle High German koppel, kopel, kuppel, feminine (masculine and neuter), ‘tie, connection,’ especially ‘leash,’ then collective ‘pack of hounds,’ also ‘band’ generally; from Latin copula, Middle Latin also cupla (the latter also ‘couple of hounds in a leash’), whence also French couple, English couple, Dutch koppel, ‘couple, multitude, troop.’