The word shell has several meanings:

1. The hard outer covering that protects certain animals. A turtle carries its shell on its back, retreating inside when danger approaches. Snails, crabs, and clams also have shells. These protective coverings are made of materials like calcium, growing slowly as the animal grows. When you walk along a beach, the beautiful shells you find are often the empty homes of creatures that once lived inside them.
2. To remove the outer covering from something. You might shell peas by popping open their pods and pushing out the round peas inside, or shell peanuts by cracking open their brittle casings.
3. In computing, a shell is a program that lets you give commands directly to a computer's operating system, like a translator between you and the machine.

People also use shell to describe someone who seems emotionally empty or withdrawn, as in “just a shell of her former self,” meaning the outer form remains but the energy and spirit have disappeared.