Abstract nonsense

Abstract nonsense is likely coin-phrased by Norman Steenrod, referring to Category Theory. On the math glossary website: Math Vault, abstract nonsense is defined as follows:

A colloquial term for category theory (a mathematical subject dedicated to the notion of category and the formalization of mathematical structures), or the methods and arguments that are related to it. 

Since different branches of mathematics deal with different mathematical structures satisfying the definition of a category, category theory can be regarded as a unifying theory of mathematics — whose results are applicable to a wide range of disciplines. In particular, if a proposition follows from abstract nonsense, then it means that its argument has more to do with the underlying categorical structure of its objects — rather than the field where it’s found. And since such arguments are often highly abstract and long-winded by nature, they are also alternatively referred to as “general abstract nonsense” or “generalized abstract nonsense“.