What does Boxing Day have to do with boxing?
1640s, noun of action, formed on model of progression, from Latin retrogressus, past participle of retrogradi "move backward" (see retrograde).
retrogression ret·ro·gres·sion (rět'rə-grěsh'ən)n.
The act or process of deteriorating or declining.
A return to a less complex biological state or stage.