What does Boxing Day have to do with boxing?
1640s, "act of struggling against," from obsolete verb reluct "to struggle or rebel against" (1520s), from Latin reluctari "to struggle against, resist, make opposition," from re- "against" (see re-) + luctari "to struggle, wrestle," perhaps shares a common origin with Greek lygos "pliant twig," lygizein "to bend, twist," Old English locc "twist of hair" (see lock (n.2)). Meaning "unwillingness" is first attested 1660s. Related: Reluctancy (1620s.).