An Etymological Dictionary of the German Language/Annotated/Hirn
Hirn, neuter, from the equivalent Middle High German hirne, Old High German hirni, neuter, ‘brain.’ We should have expected Gothic *haírni, neuter, for which hwairneins, ‘skull,’ occurs once in the genitive singular Old Icelandic hjarne, masculine, ‘brain’; also corresponding in sound to the Gothic word hvern, feminine, ‘the two white boat-shaped bones in the brain of fishes, ooliths’ (Low German has a peculiar word for Gehirn — English brain, Anglo-Saxon brœgen, Dutch brein, Middle Dutch bregen; see Brägen). The words with initial h and those with hw must be kept distinct. Dutch hersen, feminine, ‘brain’ (English dialectic harns), to which is allied Middle High German hërsenier, ‘covering for the head worn under the helmet,’ proves the origin of Old High German hirni from *hirzni, *hirsní (Old Icelandic hjarne from *hjarsne; compare Hornisse). This Old Teutonic herzn-, hersn-, is most nearly related to Sanscrit çîršn-, ‘head’ (nominative çîrša), and the closely corresponding Old Icelandic as ‘crown (of the head).’ It is also cognate with Greek κρανίον, ‘skull,’ whence results the further connection with Greek κάρα, κάρηνον, ‘head,’ Latin cerebrum (from *ceresrum), ‘brain,’ Sanscrit çiras, ‘head’; a common Aryan stem, ker, kers, ‘head,’ to which Horn is also allied. Moreover, Greek κέρνον, ‘a large earthen dish,’ might, according to the analogies mentioned under Kopf, be closely related to Hirn, ‘skull.’