necessarium

noun2

1. •
   (humorous) A lavatory.

   • Viewers […] don't mind having a hundred variations, from ‘The Necessarium’ to ‘The Thunder-Box’ to choose from. — 1967
2. •
   Synonym of reredorter: a monastery's latrine, outhouse, or lavatory, typically located behind the dormitory.

   • The other outlet from the pisalis […] leads to the necessarium, in the corner of which a light […] is kept burning. — 1848