What does Boxing Day have to do with boxing?
early 15c., "action of growing out," from Latin excrescentia (plural) "abnormal growths," from excrescentem (nominative excrescens), present participle of excrescere "grow out, grow up," from ex- "out" (see ex-) + crescere "to grow" (see crescent). Meaning "that which grows out abnormally (on a living thing) is from 1570s (excrescency in this sense is 1540s).
excrescence ex·cres·cence (ĭk-skrěs'əns)
An outgrowth from a surface that may be normal, such as a fingernail, or abnormal, such as a wart.