If you swelter, you are very uncomfortable because the weather is extremely hot. They sweltered in temperatures rising to a hundred degrees. to feel extremely hot and uncomfortable (swelt (11-20 centuries), from sweltan )
[ 'swel-t&r ] (verb.) 14th century. Middle English sweltren, frequentative of swelten to die, be overcome by heat, from Old English sweltan to die; akin to Gothic swiltan to die.