pessimum vs worst

pessimum is not a noun while worst is a noun.

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

pessimum (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

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

pessimum (verb)	worst (verb)
	defeat thoroughly

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