Synonyms for Lassitude (same or very similar meaning)
WordNet sense 1 (a state of comatose torpor (as found in sleeping sickness)):
lethargy, sluggishness
WordNet sense 2 (a feeling of lack of interest or energy):
languor, listlessness
WordNet sense 3 (weakness characterized by a lack of vitality or energy):
inanition, slackness
From the ODE community, based on WordNet
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