The Chinese idiom 不共戴天 is formed by combining four characters: 不 (not), 共 (together), 戴 (to wear), and 天 (sky). Literally, it means "not to wear the same sky together", where 戴天 expresses the idea of living under the same heaven, and 不共 negates this, resulting in a phrase that conveys an unwillingness to coexist due to intense enmity.