What does Boxing Day have to do with boxing?
late 14c., "singleness of nature, unity, indivisibility; immutability," from Old French simplicite (12c., Modern French simplicité), from Latin simplicitatem (nominative simplicitas) "state of being simple, frankness, openness, artlessness, candor, directness," from simplex (genitive simplicis) "simple" (see simplex). Sense of "ignorance" is from c.1400; that of "simplicity of expression, plainness of style" is early 15c.
Middle English also had simplesse, from French, attested in English from mid-14c. in sense "humility, lack of pride," late 14c. as "wholeness, unity;" c.1400 as "ignorance."