What does Boxing Day have to do with boxing?
early 15c., from Middle French flexible or directly from Latin flexibilis "that may be bent, pliant, flexible, yielding;" figuratively "tractable, inconstant," from flexus, past participle of flectere "to bend," of uncertain origin. Related: Flexibly.
flexible flex·i·ble (flěk'sə-bəl)
Capable of being bent or flexed.
Capable of being bent repeatedly without injury or damage.