SOS vs portent

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. 

portent

noun
  • A portending; significance 

  • Something regarded as portentous; a marvel; prodigy. 

  • Something that portends an event about to occur, especially an unfortunate or evil event; an omen. 

How often have the words SOS and portent occurred in a corpus of books? (source: Google Ngram Viewer )