1. a condition of weariness or debility : fatigue
2. a condition characterized by lack of interest, energy, or spirit : langour
Lethargy, Languor, Lassitude, Stupor, Torpor mean physical or mental inertness. Lethargy implies such drowsiness or aversion to activity as is induced by disease, injury, or drugs. Languor suggests inertia induced by an enervating climate or illness or love. Lassitude stresses listlessness or indifference resulting from fatigue or poor health. Stupor implies a deadening of the mind and senses by shock, narcotics, or intoxicants. Torpor implies a state of suspended animation as of hibernating animals but may suggest merely extreme sluggishness.
Lassitude and weariness make an interesting pair. As with many nearly synonymous pairs of words in English, one is derived from Latin and the other from Old English. Even though they both mean “the condition of being tired,” they are used in different ways. Following a common pattern, the Latinate word tends to be used in technical, medical, and formal writing, and the Old English-derived word is used when referring to physical, emotional, and spiritual qualities.
Though it is sometimes a fancy word for fatigue in medical contexts, lassitude is also used in ways that are metaphorical and closer in meaning to “negligence”
From Vocabulary.com:
A state of comatose torpor (as found in sleeping sickness)