Final answer:
Keratin is the protein that fills the outermost dead cells of the epidermis. It makes the skin tough and waterproof.
Step-by-step explanation:
The protein that fills the outermost dead cells of the epidermis is keratin. Keratin is a tough, fibrous protein that helps make the outer surface of the skin relatively tough and waterproof. It is also found in hair and nails. The stratum corneum, which is the outer layer of the epidermis, primarily consists of dead skin cells that are rich in keratin.