slackness vs lassitude

slackness and lassitude both are nouns.

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

As nouns, slackness and lassitude are synonyms defined as:

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

Other synonyms of slackness include inanition, lethargy.

slackness (noun)	lassitude (noun)
the quality of being loose (not taut)	weakness characterized by a lack of vitality or energy
the quality of being lax and neglectful	a feeling of lack of interest or energy
weakness characterized by a lack of vitality or energy	a state of comatose torpor (as found in sleeping sickness)