IPA: //ˈswɛltərɪŋli//
KK: /ˈswɛltərɪŋli/
In a way that is extremely hot and uncomfortable, often causing sweating.
The sun beat down swelteringly, making it hard to enjoy the day outside.
Swelteringly → It is formed from "swelter" (from Old English *sweltan*, meaning to die or perish from heat) and the adverbial suffix "-ingly" (meaning in a manner of). The word "swelteringly" describes a manner of being in a state of extreme heat, often to the point of discomfort or distress.
Imagine feeling so hot that you could 'perish' ('sweltan') from the heat, and the '-ingly' shows how intensely you feel that heat.