What does Boxing Day have to do with boxing?
1725, "adhering close to the surface," from Latin sessilis "pertaining to sitting, for sitting on," from sessum, past participle of sedere "to sit" (see sedentary). In botany from 1753. Meaning "sedentary" first recorded 1860.
sessile ses·sile (sěs'īl', -əl)
Permanently attached or fixed; not free-moving.