lethargic vs tardigrade

lethargic

adj

  • sluggish, slow 

  • indifferent, apathetic 

tardigrade

adj

  • Sluggish; moving slowly. 

noun

  • Sloth. 

  • A member of the animal phylum Tardigrada. 

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