smooth out, iron out, sultriness
熅 centers on applying heat to make something smooth or to create a hot, oppressive atmosphere. The idea of ironing or smoothing out comes from using warmth to remove wrinkles, while sultriness is the heavy, heated air that results.
熅 is a phono-semantic compound: 火 (fire) suggests heat, while the right side (a combination of 囚 and 皿) likely contributes the sound. The exact historical development of the right-side component is uncertain.
The left side is fire 火, and the right side looks like a person 囚 trapped over a dish 皿. Imagine using a hot iron (fire) to smooth out clothes trapped under a heavy dish, or the sultry heat of a person stuck in a small, dish-covered space.
For ウン, picture smoothing out a wrinkled 'un' (ウン) lucky charm with a warm iron. The sound 'un' cues ウン, and the heat smooths out the creases.