knock vs test the waters

knock

verb
  • To rap one's knuckles against something, especially wood. 

  • To criticize verbally; to denigrate; to undervalue. 

  • To kick a ball towards another player; to pass. 

noun
  • An abrupt rapping sound, as from an impact of a hard object against wood. 

  • A sharp impact. 

  • A batsman's innings. 

  • A blow or setback. 

  • Preignition, a type of abnormal combustion occurring in spark ignition engines caused by self-ignition; also, the characteristic knocking sound associated with it. 

  • Synonym of hunger knock 

  • A criticism. 

test the waters

verb
  • To explore or probe, as before making a commitment. 

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