slackness vs lethargy

slackness and lethargy both are nouns.

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

As nouns, slackness and lethargy are synonyms defined as:

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

Other synonyms of slackness include inanition, lassitude.

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