An Etymological Dictionary of the German Language/Annotated/schwellen
schwellen, verb, ‘to swell, rise,’ from the equivalent Middle High German swëllen, Old High German swëllan; corresponding to the equivalent Old Saxon swëllan, Dutch zwellen, Anglo-Saxon swëllan, English to swell, Old Icelandic svella; Gothic *swillan is wanting; for the Teutonic root swell, swel, see Schwiele. To this is allied the verb schwellen ‘to swell, expand’ (transitive), from Middle High German and Old High German swęllen, ‘to cause to swell,’ a factitive of schwellen. See Geschwulst.