/pes'im-l/ (Latin-based antonym for "optimal") Maximally bad. "This is a pessimal situation." Also "pessimise" To make as bad as possible. These words are the obvious Latin-based antonyms for "optimal" and "optimise", but for some reason they do not appear in most English dictionaries, although "pessimise" is listed in the OED.

[Jargon File]