gratuity vs tilly

gratuity

noun
  • An additional payment given freely as thanks for service. 

tilly

noun
  • An extra product given to a customer at no additional charge; a lagniappe. 

  • A small open-backed truck. 

adj
  • Containing till (unsorted glacial sediment). 

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