The Collaborative International Dictionary of English v.0.48:

pessimal \pes"si*mal\ adj.
   Least favorable for survival; -- of an organism's
   environment.

   Syn: pessimum(prenominal).
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

pessimal
    adj 1: of an organism's environment; least favorable for
           survival [syn: pessimal, pessimum(a)]

The Jargon File (version 4.4.7, 29 Dec 2003):

pessimal
 /pes'im?l/, adj.

    [Latin-based antonym for optimal] Maximally bad. ?This is a pessimal
    situation.? Also pessimize vt. To make as bad as possible. These words are
    the obvious Latin-based antonyms for optimal and optimize, but for some
    reason they do not appear in most English dictionaries, although
    ?pessimize? is listed in the OED.


The Free On-line Dictionary of Computing (30 December 2018):

pessimal

   /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]

   (1995-02-01)