A Lazy Class smells represent classes that are under-used.
If a class is rarely used, it can impede development by increasing the complexity of the codebase. Such a class might be unnecessary, and it’s functionality could be made a part of another class.
Lazy Class is the opposite code smell to Large Class.
Books about code smells
Learn to write better smelling code.
Refactoring: Improving the Design of Existing Code
Code smells: A Predictor of Remarkable Quality Improvement
Clean Code: A Handbook of Agile Software Craftsmanship