lassitudeEnglishOxford American Thesauruslassitude noun prolonged periods of lassitude which she ascribed to the heat: lethargy, listlessness, weariness, languor, sluggishness, enervation, tiredness, exhaustion, fatigue, sleepiness, drowsiness, torpor, torpidity, ennui, lifelessness, sloth, apathy. ▷antonyms vigor, energy.