Very hot, in the heavy, sticky way that makes you feel weak and uncomfortable. This is the key thing: 'sweltering' is not just a big number on the thermometer — it is heat you suffer in. Your shirt sticks to your back, the air feels thick, you do not want to move. So people use it most about summer days, about a room with no air conditioning, about a crowded bus. It works best with humid heat, which makes it a very natural word for Taiwan. A dry desert can be hot, and people do say 'sweltering' there too, but the word feels most at home where the air is wet. It is a normal everyday word — not formal, not slang — and it is a little dramatic, so it carries a complaint inside it. If you say 'It was sweltering', you are not giving information about the weather; you are telling someone you had a bad time. You will also see it in front of a noun a lot: a sweltering afternoon, sweltering heat, a sweltering kitchen. There is a verb 'swelter' (to suffer in the heat), but it is much rarer, and most students will only ever meet the -ing form.