n. A state of prolonged inactivity or torpor; inertness of body or mind; sluggishness; dullness; stupor.n. Specifically, in pathology, a disorder of consciousness, which consists of prolonged and profound sleep, from which the patient may be momentarily aroused, but into which he quickly sinks again.n. The hibernation or winter sleep of an animal, or any other state of complete repose, as a period of summer lethargy observed in many insect-larvæ, the repose of many tropical animals during the dry season, etc.To make lethargic or dull.n. Same as litharge.