acid test vs testing

acid test

noun

  • A rigorous test or appraisal of the quality or worth of something. 

  • In the 1960s, a party based around the use and advocacy of the psychedelic drug LSD ("acid"). 

testing

noun

  • The act of conducting a test; trialing, proving. 

adj

  • Difficult; tough. 

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