precaution vs prewarn

precaution

verb
  • To warn or caution beforehand. 

  • To take precaution against. 

noun
  • A measure taken beforehand to ward off evil or secure good or success; a precautionary act. 

  • Previous caution or care; caution previously employed to prevent misfortune or to secure good 

prewarn

verb
  • To warn beforehand; to forewarn. 

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