playing field vs threshing floor

playing field

noun
  • A field on which a game, especially a ball game, is played. 

  • Any place or context within which competitive activity takes place, such as market competition between companies. 

threshing floor

noun
  • The floor of a threshing house or similar area where grain is threshed. 

How often have the words playing field and threshing floor occurred in a corpus of books? (source: Google Ngram Viewer )