helmet
冑 centers on protective headgear: a helmet worn in battle.
冑 is thought to combine 由 (possibly representing a head) with 冃 (a cap or covering), together suggesting a helmet. The exact historical development is uncertain.
The top 由 looks like a head, and the bottom 冃 is a cap. Together they form a helmet protecting the head.
For チュウ, imagine a soldier named Chu who always wears his helmet: Chu -> チュウ, and his helmet is 冑.