cocktail vs yellow back

cocktail

verb
  • To treat (a person) to cocktails. 

  • To adulterate (fuel, etc.) by mixing in other substances. 

noun
  • A mixture of other substances or things. 

  • A mixed alcoholic beverage. 

  • A species of rove beetle, so called from its habit of elevating the tail. 

  • A horse, not of pure breed, but having only one eighth or one sixteenth impure blood in its veins. 

yellow back

noun
  • A coward. 

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