An Etymological Dictionary of the German Language/schwül
schwül, ‘sultry,’ ModHG. only, from LG. swûl; comp. Du. zwoel, ‘sultry,’ AS. swôl (Goth. *swôls, is wanting); allied, like schwelen, to OHG. swilzzôn, ‘to burn slowly,’ AS. for-swœ̂lan, ‘to burn,’ OIc. svœ́la, ‘thick, choking smoke.’ The root swē̆l, swō̆l, appears also in Lith. svìlti, ‘to smoulder,’ svilus, ‘glimmering,’ svilmis, ‘burnt smell,’ and in Lett. swelt, ‘to singe.’ Deriv. Schwulität, ‘sultriness,’ with a Lat. ending like Lappalien and Schmieralien.