worst vs pessimum

worst is a noun but pessimum is not a noun.

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

worst (noun)	pessimum (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

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

worst (verb)	pessimum (verb)
defeat thoroughly

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