What does Boxing Day have to do with boxing?
Old English heortleas "dispirited, dejected;" see heart + -less. In Middle English with expanded senses "lacking in courage; foolish; listless; half-hearted; sluggish." Sense of "callous, cruel" is not certainly attested before Shelley used it in 1816. Literal meaning "lacking a heart, lifeless" (mid-15c.) is rare. Related: Heartlessly; heartlessness.