What does Boxing Day have to do with boxing?
c.1300, from Old French entrailles (12c.), from Late Latin intralia "inward parts, intestines" (8c.), from Latin interanea, neuter plural of interaneus "internal, that which is within," from inter "between, among" (see inter-).
entrails en·trails (ěn'trālz', -trəlz)
The internal organs, especially the intestines; viscera.