bull session vs natter

bull session

noun
  • An informal discursive group discussion, often one where politics, economics or current events are discussed. 

natter

noun
  • Mindless and irrelevant chatter. 

verb
  • To nag. 

  • To talk casually; to discuss unimportant matters. 

How often have the words bull session and natter occurred in a corpus of books? (source: Google Ngram Viewer )