What does Boxing Day have to do with boxing?
late 14c., litarge, from Old French litargie or directly from Medieval Latin litargia, from Late Latin lethargia, from Greek lethargia "forgetfulness," from lethargos "forgetful," originally "inactive through forgetfulness," from lethe "forgetfulness" (see latent) + argos "idle" (see argon). The form with -th- is from 1590s in English.
lethargy leth·ar·gy (lěth'ər-jē)
A state of sluggishness, inactivity, and apathy.
A state of unconsciousness resembling deep sleep.