Origin: 1375–1425; late Middle English swelt(e)ren (v.), equivalent to swelt(en) to be overcome with heat (Old English sweltan to die; cognate with Old Norse svelta,Gothic swiltan) + -eren-er6
c.1403, frequentative of swelten "be faint (especially with heat)," c.1386, from O.E. sweltan "to die," from P.Gmc. *swel- (cf. O.S. sweltan "to die," O.N. svelta "to put to death, starve," Goth. sviltan "to die"), originally "to burn slowly," hence "to be overcome with heat or fever;" also the source