cush-cush vs tropical sweet potato

cush-cush

noun

  • Synonym of couche-couche 

  • A tropical American yam with small yellow-skinned edible tubers: Dioscorea trifida 

tropical sweet potato

noun

  • Synonym of name (yam) 

  • Synonym of white sweet potato 

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