Etymology
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acetic (adj.)

1808 (in acetic acid), from French acétique "pertaining to vinegar, sour, having the properties of vinegar," from Latin acetum "vinegar" (properly vinum acetum "wine turned sour;" see vinegar), originally the past participle of acere "be sharp; be sour" (related to acer "sharp," from PIE root *ak- "be sharp, rise (out) to a point, pierce").

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acetification (adj.)

"process of turning to vinegar," 1753, from Latin acetum "vinegar" (see acetic) + -fication, word-forming element indicating "making or causing."

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aceto- 

before vowels acet-, word-forming element from acetic and generally indicating compounds from or related to acetic acid, thus ultimately from Latin acetum "vinegar."

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acetone (n.)
colorless volatile liquid, 1839, literally "a derivative of acetic acid," from Latin acetum "vinegar" (see acetic) + Greek-based chemical suffix -one, which owes its use in chemistry to this word.
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acetylene (n.)

gaseous hydrocarbon, 1860, from French acétylène, coined by French chemist Pierre Eugène Marcellin Berthelot from chemical ending -ene + acetyl, which was coined from acetic + -yl in 1839 by German chemist Justus von Liebig. Liebig's coinage was in reference to a different radical; acetyl was transferred to its current sense in 1850s, but Berthelot's coinage was based on the original use of acetyl.

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ach (interj.)

aspirated form of ah; in English often used in representations of German or Celtic speech.

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Achaean 
in Homeric language, "a Greek," generally; later restricted to natives or inhabitants of Achaea, a region in the Peloponnesus. The Achaean League after c. 280 B.C.E. was a model for later federal republics. In Latin, Achaicus meant "a Greek."
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Achates 

name of the armor-bearer and faithful friend of Aeneas in the "Aeneid;" The phrase fidus Achates was proverbial for "faithful friend, loyal and devoted companion." The name is from Greek akhatēs "agate" (see agate).

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ache (v.)

Middle English aken, from Old English acan "suffer continued pain," from Proto-Germanic *akanan, which is perhaps from a PIE root *ag-es- "fault, guilt," with apparent cognates in Sanskrit and Greek, which itself is perhaps imitative of groaning.

Originally the verb was pronounced "ake," the noun "ache" (as in speak/speech). The noun changed its pronunciation to conform to the verb, but the spelling of both was changed to ache c. 1700 on a false assumption of a Greek origin (specifically Greek akhos "pain, distress," which rather is a distant relation of awe (n.)). Related: Ached; aching.

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ache (n.)
"continuing pain," early 15c., æche, ece "an ache, pain," from Old English æce, from Proto-Germanic *akiz, from same source as ache (v.), which see for the unusual evolution of spelling and pronunciation.
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