copilot vs seaman

copilot

noun
  • A backup or assistant pilot of an aircraft. 

verb
  • To serve as relief or assistant pilot. 

seaman

noun
  • A mariner or sailor, one who mans a ship. Opposed to landman or landsman. 

  • An enlisted rate in the United States Navy and United States Coast Guard, ranking below petty officer third class and above seaman apprentice. 

  • A person of the lowest rank in the Navy, below able seaman. 

  • A merman; the male of the mermaid. 

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