house of office

From the mostly archaic sense of office as a "duty" or "function" and hence unmentionable "bodily functions".

house of office (plural houses of office)

1. (obsolete, euphemistic) An outhouse: an outbuilding used as a lavatory.
   • 1764 August 5, David Garrick, letter:

     I never, since I left England, till now, have regal'd Myself with a good house of Office... the holes in Germany are... too round, chiefly owing... to the broader bottoms of the Germans.

   • 1823, Lord Byron, Don Juan, Canto XI, §xl, ll. 123 f.:

     The very clerks—those somewhat dirty springs
     Of office, or the House of Office.

• See Thesaurus:bathroom

• house of ease

• Oxford English Dictionary. "office, n."