lethargy (n.)

late 14c., litarge, "state of prolonged torpor or inactivity, inertness of body or mind," from Medieval Latin litargia, from Late Latin lethargia, from Greek lethargia "forgetfulness," from lethargos "forgetful," apparently etymologically "inactive through forgetfulness," from lethe "a forgetting, forgetfulness" (see latent) + argos "idle" (see argon). The form with -th- is from 1590s in English. The Medieval Latin word also is the source of Old French litargie (Modern French léthargie), Spanish and Italian letargia.