An Etymological Dictionary of the German Language/Annotated/Schwanz
Schwanz, mn., ‘tail, trail, train,’ from the equivalent Middle High German swanz, masculine (for Old High German *swanz a form zagal, Middle High German zagel, equivalent to English tail, is used). Through the medium of the intensive forms swangezen, swankzen, Middle High German swanz is connected with schwingen: Middle High German swansen, ‘to shake to and fro,’ Dutch swanselen, ‘to reel.’