An Etymological Dictionary of the German Language/Annotated/schwül
schwül, ‘sultry,’ Modern High German only, from Low German swûl; compare Dutch zwoel, ‘sultry,’ Anglo-Saxon swôl (Gothic *swôls, is wanting); allied, like schwelen, to Old High German swilzzôn, ‘to burn slowly,’ Anglo-Saxon for-swœ̂lan, ‘to burn,’ Old Icelandic svœ́la, ‘thick, choking smoke.’ The root swē̆l, swō̆l, appears also in Lithuanian svìlti, ‘to smoulder,’ svilus, ‘glimmering,’ svilmis, ‘burnt smell,’ and in Lettic swelt, ‘to singe.’ Derivative Schwulität, ‘sultriness,’ with a Latin ending like Lappalien and Schmieralien.