What does Boxing Day have to do with boxing?
c.1600, from Latin inclusionem (nominative inclusio) "a shutting up, confinement," noun of action from past participle stem of includere (see include).
inclusion in·clu·sion (ĭn-klōō'zhən)n.
A nonliving mass, such as a droplet of fat, in the cytoplasm of a cell.
The process by which a foreign or heterogenous structure is misplaced in another tissue.