leth·ar·gy / ˈle[unvoicedth]ərjē/ • n. a lack of energy and enthusiasm: periods of weakness and lethargy [in sing.] she might have sunk into a lethargy. ∎ Med. a pathological state of sleepiness or deep unresponsiveness and inactivity.

lethargy morbid drowsiness; torpor, apathy. XIV. Earliest form litargie — OF. litargie (mod. lēth-) — late L. lēthargia (medL. litargia) — Gr. lēthargíā, f. lēthargos forgetful, f. *lēth- (cf. prec.), var. of *lath- in lanthánein escape notice, lanthánesthai forget, prob. rel. to L. latēre be hid.
So lethargic (XIV rare before XVI.)— L. — Gr.

lethargy (leth-er-ji) n. mental and physical sluggishness: a degree of inactivity and unresponsiveness approaching or verging on the unconscious.