What does Boxing Day have to do with boxing?
early 15c., "something sent forth," from Middle French émission (14c.) and directly from Latin emissionem (nominative emissio) "a sending out, projecting, hurling, letting go, releasing," from past participle stem of emittere "send out" (see emit). Meaning "a giving off or emitting" is from 1610s.
emission e·mis·sion (ĭ-mĭsh'ən)
A discharge of fluid from a living body, usually a seminal discharge.