from The Century Dictionary and Cyclopedia
- n. A commode or close-stool for use at night, as in a bedroom.
Sorry, no etymologies found.
But if he wish to evacuate the bowels, let him do so upon a very narrow night-stool.
BREASTPLATE-MAKER So you would pay ten minae (1) for a night-stool?
TRYGAEUS 'Twould be very useful as a night-stool ...
So you would pay ten minae  for a night-stool?
Keep the night-stool and urinal ready for whenever he calls, and take it back when done with.
For this purpose, we filled, over night, a large chest and a night-stool, with billets of wood, rubbish, stones, and other useless matters, to make them heavy, binding them up carefully with mats and ropes to give them an air of importance.
A General History and Collection of Voyages and Travels — Volume 09 Arranged in Systematic Order: Forming a Complete History of the Origin and Progress of Navigation, Discovery, and Commerce, by Sea and Land, from the Earliest Ages to the Present Time
Let me not become the night-stool of the comic poets. [