An Etymological Dictionary of the German Language/Annotated/Hahnrei
Hahnrei, masculine, ‘cuckold,’ Modern High German only; of obscure origin; in earlier German it signifies ‘capon.’ Its figurative sense, ‘cuckold,’ derived from ‘capon,’ agrees with the expression Hörner tragen, literally ‘to wear horns.’ Formerly the spur was frequently cut of and placed as a horn in the comb; the hoodwinked husband is thus compared to a capon. On account of the earlier variant Hahnreh, we may regard Hahnrei as a compound of Reh.