Sweltering means uncomfortably hot. Walking home from work on a sweltering day will leave you sweating.
A sweltering summer afternoon might cause you to turn on the air conditioning in your apartment or take a cool shower. To be sweltering is more than merely being hot — it's the kind of damp, intense heat that sends everyone to the pool or beach for some relief. The verb swelter came first, from the now obsolete word swelten, "be faint from heat," rooted in the Old English sweltan, "to die or perish."