old hat vs permutation

old hat

adj
  • Very familiar; common, hackneyed or out of date. 

permutation

noun
  • A transformation of a set's prime form, by applying one or more of certain operations, specifically, transposition, inversion, and retrograde. 

  • One of the ways something exists, or the ways a set of objects can be ordered. 

  • An ordering of a finite set of distinct elements. 

  • A one-to-one mapping from a finite set to itself. 

How often have the words old hat and permutation occurred in a corpus of books? (source: Google Ngram Viewer )