What does Boxing Day have to do with boxing?
mid-15c., sompnolent, from Old French sompnolent (Modern French somnolent) or directly from Latin somnolentus "sleepy, drowsy," from somnus "sleep" (see Somnus). Respelled 17c. on Latin model.
somnolent som·no·lent (sŏm'nə-lənt)adj.
Inducing or tending to induce sleep; soporific.
In a condition of incomplete sleep; semicomatose.