肌肤 (jī fū) is a literary, poetic word for human skin, common in beauty or skincare contexts — not the everyday word. Use 皮肤 (pí fū) for general skin (human, animal, fruit) and 皮 (pí) for animal hides/leather or the adjective 'naughty.' 肌肤 never refers to animal skin or fruit peel.