#1 lethargy n. [lassitude, sluggishness] a state of comatose torpor (as found in sleeping sickness). wn #2 lethargy n. [inanition, lassitude, slackness] weakness characterized by a lack of vitality or energy. wn #3 lethargy n. [languor, sluggishness, phlegm, flatness] inactivity; showing an unusual lack of energy; "the general appearance of sluggishness alarmed his friends". wn