turn back vs turn under

turn back

verb
  • To cause to reverse direction and retrace one's steps. 

  • To return to a previous state of being. 

  • To reverse direction and retrace one's steps. 

  • To prevent or refuse to allow passage or progress. 

  • To adjust to a previous setting. 

  • To fold something back; to fold down. 

turn under

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