To suffer in heat — to be so hot that you feel weak, sticky and miserable. It is not just a measure of temperature; it always carries the discomfort. You swelter in a car with no air conditioning, in a crowded train, in a suit at an outdoor wedding in July. It is used with in: they sweltered in the afternoon sun. Honestly, the plain verb is not the form you will meet most. Far more common is the adjective sweltering, which describes the weather itself — a sweltering afternoon, sweltering heat, it's sweltering out there — and that one you will hear all the time in summer. There is a noun swelter for the heat itself, but it is rare and literary. The word has a formal, written feel; a friend texting you about the weather is more likely to write 'it's boiling' or 'it's roasting'. Newspaper headlines love it: 'City swelters as temperatures hit 38'.

熱得受不了、悶熱難耐（重點在「難受」，不只是溫度高）。台灣夏天那種又濕又悶、走兩步就滿身汗的感覺就是 sweltering。