smallest room vs thunderbox

smallest room

noun
  • The lavatory: the room intended for urination and defecation. 

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 smallest room and thunderbox occurred in a corpus of books? (source: Google Ngram Viewer )