heavy metal vs refrain

heavy metal

noun
  • Great influence or power. 

  • A genre descended from rock music, characterized by massive sound, highly amplified distortion, and overall loudness, often with extended guitar solos, and lyrics that involve aggressive or fantastic imagery. 

  • Any metal that has a specific gravity greater than about 5, especially one, such as lead, that is poisonous and may be a hazard in the environment. (There are many different definitions of what counts as a heavy metal; see Heavy metals for a discussion.) 

  • guns or shot of large size. 

refrain

noun
  • A much repeated comment, complaint, or saying. 

  • The chorus or burden of a song repeated at the end of each verse or stanza. 

verb
  • To stop oneself from some action or interference; to abstain. 

  • To abstain from (food or drink). 

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