keep mum vs open up

keep mum

verb
  • To refrain from talking, especially to remain silent about something that may be sensitive or secret. 

open up

verb
  • Synonym of cheat out (“to face or turn toward the audience more than would be natural”) 

  • To begin running, driving, travelling, etc., at maximum speed. 

  • To commence firing weapons. 

  • To rain. 

  • To open. 

  • To widen. 

  • To reveal oneself; to share personal information about oneself; to become communicative. 

How often have the words keep mum and open up occurred in a corpus of books? (source: Google Ngram Viewer )