serf vs third 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. 

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 serf and third string occurred in a corpus of books? (source: Google Ngram Viewer )