betrothen vs significant other

betrothen

noun

  • One who is betrothen or betrothed, i.e. a fiancé or fiancée. 

adj

  • Betrothed 

significant other

noun

  • A person with whom one has an important bond of some kind. 

  • One's exclusive romantic partner. 

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