keep mum vs tip off

keep mum

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

tip off

verb
  • To begin. 

  • To alert or inform (someone), especially confidentially. 

  • To use a particular brush technique for producing a flawless surface on a wet coat of paint. 

  • To put the ball in play by throwing it up between two opponents. 

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