lethargy vs slackness

lethargy and slackness both are nouns.

Word	Noun	Adjective	Verb	Adverb
lethargy	Yes	No	No	No
slackness	Yes	No	No	No

As nouns, lethargy and slackness are synonyms defined as:

lethargy and slackness: weakness characterized by a lack of vitality or energy

Other synonyms of lethargy include inanition, lassitude.

lethargy (noun)	slackness (noun)
weakness characterized by a lack of vitality or energy	the quality of being loose (not taut)
inactivity; showing an unusual lack of energy	the quality of being lax and neglectful
a state of comatose torpor (as found in sleeping sickness)	weakness characterized by a lack of vitality or energy