acoustic: [17] Appropriately enough, acoustic may be distantly related to hear. It first appeared in English in Francis Bacon’s Advancement of Learning 1605, borrowed from Greek akoustikós. This in turn was derived from the Greek verb for ‘hear’, akoúein, which, it has been speculated, may have some connection with *khauzjan, the original Germanic source of English hear, not to mention German hören and Dutch horen (as well as with Latin cavēre ‘be on one’s guard’, and hence with English caution and caveat). => caution, caveat, hear
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.
