lassitude vs lethargy

lassitude and lethargy both are nouns.

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

As nouns, lassitude and lethargy are synonyms defined as:

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

Other synonyms of lassitude include inanition, slackness.

As nouns, lassitude and lethargy are synonyms defined as:

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

Other synonyms of lassitude include sluggishness.

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