Definition - the least favorable environmental condition under which an organism can survive
Pessimum is often found used in scientific contexts, but that doesn't mean you may not employ it in a variety of broadened or figurative senses. The word comes from the Latin pessimus ("worst"), and has been in use, and applicable to describe the condition of your child's room, since the early 19th century.
Houses dilapidated and deserted; fences gone, fields grown up in broom grass and pine sapplings; people grown scarcer; wild animals more plenty; and the cultivation of the earth, reduced, I would say to the minimum and the pessimum of human subsistence.
— The Fayetteville Observer (Fayetteville, NC), 5 Jul. 1826