The word "脸蛋" is formed by combining the character "脸", meaning "face", with "蛋", meaning "egg", creating a literal compound that translates to "face egg". This construction relies on a vivid metaphorical association where the round, smooth, and delicate qualities of an egg are used to evoke the soft, plump appearance of human cheeks, thereby specifying and enriching the concept of the face with a colloquial and often affectionate nuance focused on the cheeks or facial complexion.