What does Boxing Day have to do with boxing?
c.1600, from French acoustique, from Greek akoustikos "pertaining to hearing," from akoustos "heard, audible," verbal adjective from akouein "to hear," probably from copulative prefix a- + koein "to mark, perceive, hear," from PIE *kous- "to hear," perhaps from root *(s)keu- "to notice, observe" (see caveat). Acoustic guitar (as opposed to electric) attested by 1958. Related: Acoustical; acoustically.
acoustic a·cous·tic (ə-kōō'stĭk) or a·cous·ti·cal (-stĭ-kəl)
Of or relating to sound, the sense of hearing, or the perception of sound.