SOS vs tip off

SOS

noun
  • A children's game in which players take turns to place S's and O's on a grid, collecting points by creating an "SOS" sequence. 

  • The conventional Morse code call made by a ship in distress. 

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