past master vs pro

past master

noun
  • One who has an extremely high level of ability or knowledge within an area of achievement or expertise. 

  • Someone who has previously been a master at a given lodge. 

pro

noun
  • A person who supports a concept or principle. 

  • Professional. 

  • A prostitute. 

  • A professional sportsman. 

  • A chemical prophylaxis taken after sex to avoid contracting venereal disease. 

  • An advantage of something, especially when contrasted with its disadvantages (cons). 

prep
  • In favor of. 

adj
  • Professional. 

How often have the words past master and pro occurred in a corpus of books? (source: Google Ngram Viewer )