onomastics vs tropical sweet potato

onomastics

noun
  • The branch of lexicology devoted to the study of names and naming, especially the origins of names. 

tropical sweet potato

noun
  • Synonym of name (yam) 

  • Synonym of white sweet potato 

How often have the words onomastics and tropical sweet potato occurred in a corpus of books? (source: Google Ngram Viewer )