What does Boxing Day have to do with boxing?
"copulation," 1713, scientific Latin, from Latin coitus "a meeting together; sexual union," past participle of coire, from com- "together" (see co-) + ire "come, go," (see ion). In Middle English as coite (early 15c.). Used in English in general senses of "meeting, uniting," and also of magnetic force, planetary conjunction, etc., before sexual sense came to predominate.
coitus co·i·tus (kō'ĭ-təs, kō-ē'-)
Sexual union between a male and a female involving insertion of the penis into the vagina. Also called coition, copulation.