be angry (excited)
慍 centers on a heated, agitated anger: being angry in an excited or resentful way.
慍 combines 忄 (heart/mind) with 昷 (囚 over 皿), which likely contributes the sound and suggests a contained, heated feeling. The exact historical development is uncertain.
The heart radical 忄 beside a shape that looks like a person 囚 trapped over a dish 皿: imagine anger simmering like a trapped person rattling a dish in frustration.
For ウン, picture the angry person letting out a low, rumbling 'unnn' sound of irritation: 'unnn' -> ウン, the sound of contained anger.