What does Boxing Day have to do with boxing?
1620s, back-formation from expurgation or from Latin expurgatus, past participle of expurgare "to cleanse out, purge, purify" (see expurgation). Related: Expurgated; expurgating. The earlier verb was simply expurge (late 15c.), from Middle French expurger.
To clean up, remove impurities. An expurgated edition of a book has had offensive words or descriptions changed or removed.