house of office

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

Noun

 * 1)  An outhouse: an outbuilding used as a lavatory.
 * 2) * 1660 20 October, Samuel Pepys, diary:
 * Going down my cellar to look, I put my foot into a heap of turds, by which I find that Mr Turner’s house of office is full and comes into my cellar, which doth trouble me; but I will have it helped.
 * 1) * 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.

Synonyms

 * See Thesaurus:bathroom