shellback vs thalassoid

shellback

noun

  • A sailor who has crossed the equator. 

  • A worldly sailor. 

thalassoid

noun

  • Any small lunar "sea" 

  • Any moth of the genus Thalassodes 

adj

  • Synonym of marine 

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