tern vs threesome

tern

noun
  • A thing with three components; a set of three things. 

  • Any of various seabirds of the subfamily Sternidae (of the family Laridae) that are similar to gulls but are smaller and have a forked tail. 

adj
  • Consisting of three components; ternate, threefold, triple. 

threesome

noun
  • A group of three people or things. 

  • An instance of sexual activity involving three people. 

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