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- (see a-(3)) + koein "to mark, perceive, hear," from PIE *kous- "to hear," which is perhaps from root *(s)keu- "to notice, observe" (see caveat). Acoustic guitar (as opposed to electric) attested by 1958. Related: Acoustical; acoustically.