Lethargic means lacking energy, sluggish or drowsy.
Lethargic is an adjective.
Synonyms: listless, lifeless, idle, enervated, languid, sleepy, slothful, sluggish, indolent.
Antonyms: alert, active, spirited, animated, vigorous, energetic.
Something must be very wrong if the two-year old boy is acting lethargic.
The extreme heat outside is making the entire students lethargic.
Mr.Ravi's profound lethargy made a delay in his project work.
Most people feel lethargic after eating a large stomach filling dinner.