What does Boxing Day have to do with boxing?
late 15c., past participle adjective from truncate. Originally in heraldry; modern senses are post-1700.
truncate trun·cate (trŭng'kāt')
v. trun·cat·ed, trun·cat·ing, trun·cates
To shorten by or as if by cutting off, especially by cutting across at right angles to the long axis.