conceal vs tip off

conceal

verb
  • To hide something from view or from public knowledge, to try to keep something secret. 

tip off

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

  • To begin. 

  • 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 conceal and tip off occurred in a corpus of books? (source: Google Ngram Viewer )