Definition of lethargic:

deficient in alertness or activity

Synonyms of lethargic:

unenergetic, sluggish, listless, lazy, inactive, torpid, slow, sleepy, slothful, apathetic

Opposite/Antonyms of lethargic:

energetic, active, busy, lively, perky, alert, vivacious, vigorous, vital, animated

Sentence/Example of lethargic:

If the cat is lethargic and his coat is dull and unhealthy looking, this too may be a sign.