pessimal vs worst

pessimal is not a noun while worst is a noun.

Word	Noun	Adjective	Verb	Adverb
pessimal	No	Yes	No	No
worst	Yes	Yes	Yes	Yes

pessimal (noun)	worst (noun)
	the weakest effort or poorest achievement one is capable of
	the greatest damage or wickedness of which one is capable
	the least favorable outcome

pessimal (adjective)	worst (adjective)
of an organism's environment; least favorable for survival	(superlative of `bad') most wanting in quality or value or condition

pessimal (verb)	worst (verb)
	defeat thoroughly

pessimal (adverb)	worst (adverb)
	to the highest degree of inferiority or badness