helper vs third string

helper

noun
  • That which helps; anything serving to assist. 

  • One who helps; an aide; assistant; auxiliary. 

  • A locomotive that assists a train, usually on steep gradients. 

  • A person who does cleaning and cooking in a family home, or in a market; domestic employee. 

third string

adj
  • Of a decidedly lower quality or condition. 

noun
  • A unit of players that plays behind the first and second strings; a junior varsity team. 

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