What does Boxing Day have to do with boxing?
1520s, from Late Latin phthisis "consumption," from Greek phthisis "wasting, consumption; perishing, decay; waxing," from phthiein "to decay, waste away," from PIE root *dhgwhei- "to perish, die away" (cf. Sanskrit ksitih "destruction," ksinati "perishes").
phthisis phthi·sis (thī'sĭs, tī'-) or phthis·ic (tĭz'ĭk, thĭz'-)
A disease characterized by the wasting away or atrophy of the body or a part of the body.
Tuberculosis of the lungs. No longer in technical use.