intern vs keep under lock and key

intern

noun
  • A student or recent graduate who works in order to gain experience in their chosen field. 

  • A medical student or recent graduate working in a hospital as a final part of medical training. 

  • A person who is interned, forcibly or voluntarily. 

verb
  • To work as an intern. Usually with little or no pay or other legal prerogatives of employment, for the purpose of furthering a program of education. 

  • To imprison somebody, usually without trial. 

  • To internalize. 

  • To confine or hold (foreign military personnel who stray into the state's territory) within prescribed limits during wartime. 

keep under lock and key

How often have the words intern and keep under lock and key occurred in a corpus of books? (source: Google Ngram Viewer )