lassitude vs slackness

lassitude and slackness both are nouns.

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

As nouns, lassitude and slackness are synonyms defined as:

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

Other synonyms of lassitude include inanition, lethargy.

lassitude (noun)	slackness (noun)
weakness characterized by a lack of vitality or energy	the quality of being loose (not taut)
a feeling of lack of interest or 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