adjective (a)

• deficient in alertness or activity(adj.all)

  Example:
  Bullfrogs became lethargic with the first cold nights.
  

  Synonym:
  unenrgetic
  Antonym:
  energetic
  

  source: wordnet30
• Pertaining to, affected with, or resembling, lethargy; morbidly drowsy; dull; heavy.(adjective)

  source: webster1913