What does Boxing Day have to do with boxing?
1620s, "declare, express," from Latin enuntiatus, past participle of enuntiare "speak out, say, express, assert; divulge, disclose, reveal, betray," from ex- "out" (see ex-) + nuntiare "to announce" (see nuncio). Or perhaps a back-formation from enunciation. Meaning "to articulate, pronounce" is from 1759. Related: Enunciated; enunciating.