mutation vs old hat

mutation

noun
  • An alteration a particular sound of a word, especially the initial consonant, which is triggered by the word's morphological or syntactic context and not by its phonological context. 

  • A group of thrushes. 

  • Any alteration or change. 

  • A mutant. 

  • Any heritable change of the base-pair sequence of genetic material. 

old hat

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  • Very familiar; common, hackneyed or out of date. 

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