lethargy vs lassitude

lethargy and lassitude both are nouns.

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

As nouns, lethargy and lassitude are synonyms defined as:

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

Other synonyms of lethargy include inanition, slackness.

As nouns, lethargy and lassitude are synonyms defined as:

lethargy and lassitude: a state of comatose torpor (as found in sleeping sickness)

Other synonyms of lethargy include sluggishness.

lethargy (noun)	lassitude (noun)
weakness characterized by a lack of vitality or energy	weakness characterized by a lack of vitality or energy
inactivity; showing an unusual lack of energy	a feeling of lack of interest or energy
a state of comatose torpor (as found in sleeping sickness)	a state of comatose torpor (as found in sleeping sickness)