From Merriam-Webster Dictionary
1. : of, relating to, or characterized by laziness or lack of energy : feeling or affected by lethargy : sluggish
2: indifferent, apathetic
Ex 1. Today, after flying to the West Coast from the East Coast for a day, I’m feeling lethargic.
Ex 2. To his parents, the boy acted lethargic about his new computer, but inwardly he was thrilled.