second string vs serf

second string

adj
  • Not as good; of a lower quality or condition. 

noun
  • A unit of players which is considered to have a secondary level of talent and which usually plays after the first string. 

serf

noun
  • A partially free peasant of a low hereditary class, attached like a slave to the land owned by a feudal lord and required to perform labour, enjoying minimal legal or customary rights. 

  • A worker unit. 

  • A similar agricultural labourer in 18th and 19th century Europe. 

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