be angry
瞋 centers on the expression of anger, specifically a fierce glare or angry look directed at someone.
瞋 combines 目 (eye) with 真, which likely contributes the sound and may relate to the idea of fullness or intensity, suggesting eyes filled with anger.
The left side is an eye 目, and the right side 真 looks like a straight, intense stare. Picture an eye glaring with such force that it feels like a piercing truth: an angry look.
For シン, imagine a person so angry their glare could cut like a shin guard: shin -> シン, and the eye 目 is fixed in fury.