Extremely hot and uncomfortable, or the state of being uncomfortably hot.
From 'swelter', which may come from Old English 'sweltan' meaning to faint or die, influenced by 'sweat'. The word took on its modern meaning of being intensely hot in the 1600s.
Sweltering originally meant literally dying from heat—'sweltan' is an Old English word for death, so when you say it's sweltering, you're using language that once described an actual threat, now just describing discomfort.
Complete word intelligence in one call. Free tier — 50 lookups/day.