Etymology
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hear (v.)

Old English heran (Anglian), (ge)hieran, hyran (West Saxon) "to hear, perceive by the ear, listen (to), obey, follow; accede to, grant; judge," from Proto-Germanic *hausejanan (source also of Old Norse heyra, Old Frisian hera, hora, Dutch horen, German hören, Gothic hausjan "to hear"), from PIE root *kous- "to hear" (source also of Greek koein "to mark, perceive, hear;" see acoustic). The shift from *-s- to -r- is a regular feature in some Germanic languages. For the vowels, see head (n.).

Spelling distinction between hear and here developed 1200-1550. Meaning "be told, learn by report" is from early 14c. Old English also had the excellent adjective hiersum "ready to hear, obedient," literally "hear-some" with suffix from handsome, etc. Hear, hear! (1680s) originally was imperative, an exclamation to call attention to a speaker's words ("hear him!"); now a general cheer of approval. To not hear of "have nothing to do with" is from 1754.

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hearer (n.)
mid-14c., agent noun from hear.
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heard 
past tense and past participle of hear, Old English herde. To have heard of "know about" is from 1907.
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hearing (n.)
early 13c., "perception of sound by ear, action of listening," verbal noun from hear (v.). Meaning "a listening to evidence in a court of law" is from 1570s. Hearing-aid is from 1908.
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mishear (v.)

c. 1200, misheren, "to hear or listen to (sinful talk)," from mis- (1) "badly, wrongly" + hear (v.). Sense of "to hear incorrectly, mistake in hearing" is attested by mid-13c. Related: Misheard; mishearing. Old English mishieran, mishyran meant "to disobey."  

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rehear (v.)

also re-hear, "to hear again," 1680s, originally legal, "to try (a cause) a second time," from re- "again" + hear. Of sounds, by 1799. Related: Reheard; rehearing. As a noun, rehearing "a second hearing" is from 1680s.

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hearken (v.)
late Old English heorcnian "to give ear, listen" (intransitive); hear with attention" (transitive), a suffixed form of *heorcian (root of hark); from Proto-Germanic *hausjan (see hear). Harken is the usual spelling in U.S. and probably is better justified by etymology; OED says preference for hearken in British use likely is from influence of hear.
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hearsay (n.)
"information communicated by another, gossip," mid-15c., from phrase to hear say (Middle English heren seien, Old English herdon secgan). The notion is "hear (some people) say;" from hear (v.) + say (v.). As an adjective from 1570s. Hearsay evidence (1670s) is that which the witness gives not from his own perception but what was told to him. Compare similar formation in Dutch hooren zeggen, German hörensagen.
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unheard (adj.)
c. 1300 "not detected by sense of hearing," from un- (1) "not" + past participle of hear. Meaning "unknown, new" is attested from late 14c. (Old English had ungehered in this sense). Usually with of since 1590s. Similar formation in Old Norse oheyrðr, Danish uhørt, Middle Dutch ongehoort, Old High German ungehoret.
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acoustic (adj.)
c. 1600, "pertaining to hearing or sound," from French acoustique, from Latinized form of 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 root *kous- "to hear" (also source of English hear).

In reference to material meant to deaden sound, 1924. Of sound reproduced mechanically (rather than electrically) from 1932 in reference to gramophone players; acoustic guitar (as opposed to electric) attested by 1958. Related: Acoustical; acoustically.
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