The word 未婚妻 is formed by directly combining three meaningful characters to create a new, specific concept. It starts with 未 (wèi), meaning "not yet", which negates the idea that follows. This is applied to 婚 (hūn), meaning "marriage", creating the concept of "not yet married". Finally, 妻 (qī), meaning "wife", is attached. Therefore, the entire term logically builds the idea of a "not-yet-married wife", which corresponds perfectly to the English concept of "fiancée".