propose marriage vs tip off

propose marriage

verb
  • To ask someone if they want to become one's spouse. 

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