acousticadj.n.

Etymology: < post-classical Latin acusticus, acousticus (1600 in nervus acusticus ; earlier (1547) in porus acusticus (after Hellenistic Greek πόρος ἀκουστικός orifice of the ear); earlier (1541) as noun denoting a disciple of Pythagoras; also used as a Greek word in the Latin author Aulus Gellius (2nd cent. a.d.)) < ancient Greek ἀκουστικός of or for hearing < ἀκουστός audible ( < ἀκουσ- , stem of ἀκούειν to hear, probably < the same Indo-European base as hear v. + -τός , suffix forming verbal adjectives) + -ικός -ic suffix. Compare French acoustique (1700 as noun denoting the science of acoustics, 1701 as adjective), Spanish acústico (1709), Italian acustico (1679).With acoustic instrument (see sense A. 1b) compare post-classical Latin instrumentum acusticum (1665 or earlier). With the use as noun compare post-classical Latin acoustica acoustic art (1623 in a British source). N.E.D. (1884) also gives the pronunciation (ăkɑu·stik) /əˈkaʊstɪk/. Until the 1880s, this was the sole pronunciation given in dictionaries. From then on both pronunciations were given, with priority shifting to the monophthongal variant during the early 20th cent. In the late 20th cent. the diphthongal variant, when given, was labelled as old-fashioned.