What does Boxing Day have to do with boxing?
late 14c., "hard-working, industrious," from Old French laborios "arduous, wearisome; hard-working" (12c., Modern French laborieux), from Latin laboriosus "toilsome, wearisome, troublesome," from labor (see labor (n.)). Meaning "costing much labor, burdensome" is from early 15c.; meaning "resulting from hard work" is mid-15c. Related: Laboriousness.