Several river names in Germany (e.g., Inn , in antiquity Aenus ) and Austria (e.g., Enns , in antiquity and the Middle Ages Anisa , Anisus ), but also in Lithuania (e.g. Anatà , Anýkšté ) and in Ukraine ( Onut ) were traditionally analyzed as continuing the ‘hydronymic root PIE * en ‐/* on ‐’. This root, however, is a ghost-root, even if it is reconstructed according to current conventions as PIE * h 1 en ( H )‐. The Central European names are most probably of Celtic origin and go back to PIE * pen ‐ ‘swamp, slowly flowing water’. It became obvious that another root might play a role in this context as well, namely PIE * h 2 en ‐ ‘to haul (water)’: This root might theoretically be the basis of all semantically fitting appellatives whose preform has an onset PCelt. * an ‐ (< PIE * h 2 en ‐), PGerm./PBalt./PSlav. * an ‐ (< PIE * h 2 en ‐/* h 2 on ‐) or PCelt. * on ‐ (< PIE * h 2 on ‐). This reconstruction would also work for names mentioned above, Germ. Enns / Anisa , Lith. Anatà , Anýkšté , Ukrain. Onut. And it might also explain Gaul. Anam ‘swamp’ and MIr. an f. ‘water, urine’.
Harald Bichlmeier (Thu,) studied this question.