An Etymological Dictionary of the German Language/Jahn
Jahn, m., ‘swath,’ first found in early ModHG., yet undoubtedly a genuine G. word, existing throughout South Germany (MidHG. *jân), and also appearing in Swed. dials. as ån. In Swiss dials. Jahn means ‘passage (formed by a swath)' Hence the word is a derivative of the Aryan root yê, or rather ĭ, ‘to go,’ with which Goth. iddja, ‘went’ (Sans. yâ, ‘to go’), is connected. See gehen and eilen.