The Chinese word "同居" is formed by directly combining the character "同", meaning "together" or "same", with the character "居", meaning "to live" or "reside". This combination creates a literal sense of "living together", which encapsulates the concept of cohabitation. The formation follows a straightforward semantic blending where the individual meanings of the characters merge to convey a unified idea, exemplifying how many Chinese words are constructed by pairing characters to express compound concepts.