aposiopesis vs echoism

aposiopesis

noun
  • An abrupt breaking-off in speech, often indicated in print using an ellipsis (…) or an em dash (—). 

echoism

noun
  • The practice of creating words or language by imitating sounds from the environment. 

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