There are several synonyms for the phrase "thick shell." Some of the most common include "hard exterior," "tough outer layer," "dense covering," "sturdy casing," and "robust armor." These phrases are often used to describe a physical object or living creature with a strong, protective outer layer. In some cases, they may also be used more metaphorically to describe a person or organization with a resilient, unyielding demeanor. Whatever the context, these synonyms convey a sense of strength, durability, and resilience that can be useful in a wide range of writing and speech situations.