What does Boxing Day have to do with boxing?
c.1600, perhaps from French filtration (1570s), noun of action from filter "to filter" (see filter (v.)).
filtration fil·tra·tion (fĭl-trā'shən)
The process of passing a liquid through a filter.
filtrate fil·trate (fĭl'trāt')
v. fil·trat·ed, fil·trat·ing, fil·trates
To put or go through a filter. n.
Material, especially liquid, that has passed through a filter.