Definition: the least favorable environmental condition under which an organism can survive
Pessimum is admittedly scientific jargon, but sometimes you need a boldly scientific word to give your proclamations that soupçon of gravitas when you aren't doing so well. The word comes to English straight from Latin. If you need a fancy adjective for "worst" to go with this noun, you can use pessimal.
Suppose that the British Government, instead of being anxious, as they are, to vomit forth the Lazaroni of Ireland—that wretched population who are reduced to the minimum, and the pessimum too, of human existence—to the potato, the whole potato, and nothing but the potato—was desirous of detaining them in Ireland … but would it be “inhuman” … to bring a number of these miserable beings to this country!
—Register of Debates in Congress, 10 Jan. 1828