What does Boxing Day have to do with boxing?
mid-14c., "firmness of resolve, mental equilibrium" (of persons), from Old French stableté, from Latin stabilitatem (nominative stabilitas) "firmness, steadfastness," from stabilis "steadfast, firm" (see stable (adj.)). In physical sense, "difficult to overthrow," it is recorded from early 15c.
stability sta·bil·i·ty (stə-bĭl'ĭ-tē)
The condition of being stable or resistant to change.