A Middle Man is a class that in responsible, principally, for delegation.
A class with a Middle Man smell increases the complexity of the code without contributing the program’s function, as well as allows code to avoid logical flows of data based the relationship between classes.
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