OED gives the definition and a quote from 1833 as the earliest reference as:
The first week-day after Christmas-day, observed as a holiday on which post-men, errand-boys, and servants of various kinds expect to receive a Christmas-box. So also Boxing-night, Boxing-time.
1833 in A. Mathews Mem. C. Mathews (1839) IV. viii. 173 To the completion of his dismay, he arrives in London on boxing-day.
According the Etymonline, it goes back as early as 1809:
1809, "first weekday after Christmas," on which postmen and others expect to receive a Christmas present, originally in reference to the custom of distributing the contents of the Christmas box, which was placed in the church for charity collections. See box (n.1). The custom is older than the phrase
However, most of the other sources say that the origin is unclear and list several possible theories.
Wikipedia states that the exact etymology of the term "boxing day" is unclear. There are several competing theories, none of which is definitive. The European tradition, which has long included giving money and other gifts to those who were needy and in service positions, has been dated to the Middle Ages, but the exact origin is unknown. It is believed to be in reference to the Alms Box placed in places of worship to collect donations to the poor.
Britishfood.about.com lists four possible origins and says that it might be related to all of them:
- A ‘Christmas Box’ in Britain is a name for a Christmas present.
- Boxing Day was a day off for servants and when they received a ‘Christmas Box’ from the master. The servants would also go home to give ‘Christmas Boxes’ to their families.
- A box to collect money for the poor was placed in Churches on Christmas day then opened the next day.
- Great sailing ships when setting sail would have a sealed box containing money on board for good luck.If the voyage were a success the box was given to a priest, opened at Christmas and the contents given to the poor.
Finally, snopes.com debunks most of the theories and gives the following origin as the best bet:
The holiday's roots can be traced to Britain, where Boxing Day is also known as St. Stephen's Day. Reduced to the simplest essence, its origins are found in a long-ago practice of giving cash or durable goods to those of the lower classes. Gifts among equals were exchanged on or before Christmas Day, but beneficences to those less fortunate were bestowed the day after.
And that's about as much as anyone can definitively say about its origin because once you step beyond that point, it's straight into the quagmire of debated claims and dueling folklorists. Which, by the way, is what we're about to muddy our boots with.
Well, it looks like I already answered the question but different sources say different things. I trust the users on this site who have advanced searching skills and can find a definitive answer.
What is the exact origin of this phrase? Can we find a better answer based on the first usages of this phrase?