Synonyms for Lethargy (same or very similar meaning)
WordNet sense 1 (a state of comatose torpor (as found in sleeping sickness)):
lassitude, sluggishness
WordNet sense 2 (weakness characterized by a lack of vitality or energy):
inanition, slackness
WordNet sense 3 (inactivity; showing an unusual lack of energy):
flatness, languor, phlegm
From the ODE community, based on WordNet
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