The word 皮肤 is formed by combining two characters, each representing a closely related aspect of the same concept. The first character, 皮 (pí), originally referred to the outer layer or hide of an animal. The second character, 肤 (fū), specifically denotes the surface layer of a human body. When placed together, they create a single, more comprehensive term where the meanings merge and reinforce each other, resulting in the general and complete concept of "skin" encompassing both its surface and its substance.