ladies' room vs thunderbox

ladies' room

noun
  • A public lavatory intended for use by women. 

  • A room belonging to or intended for some or all women 

  • A waiting room in a railway station or other public building intended exclusively for women and usually including a separate women's lavatory. 

thunderbox

noun
  • Any lavatory or toilet, especially a rudimentary outdoor latrine or toilet, or an outhouse. 

  • A box of metal balls which is shaken to create a thunder sound effect. 

  • A chamber pot enclosed in a box; a portable commode. 

How often have the words ladies' room and thunderbox occurred in a corpus of books? (source: Google Ngram Viewer )