When you are lethargic, your body feels heavy and slow, and your mind does not want to start anything. It is more than a bit tired. Tired can lift after sleep. Lethargic is that thick, dull state where even a small job feels far away — after illness, in heavy heat, or when you have not slept for nights. Doctors use it too: a lethargic patient is worryingly low in energy, not merely uninterested. Lazy is a judgement about effort. Lethargic is a state you are in; you would move if the body had anything left. The word is a little formal — you meet it in writing and in medical talk — but educated speakers use it in ordinary conversation as well. It comes from Greek lēthargikós, from lḗthargos “forgetful, sluggish”: lēthē “forgetting” (the same root as the river Lethe, whose water made the dead forget their lives) plus argós “not working.” In English you can also see it as lethargy + -ic.