1580s, fabled river of the Lower World in Greek mythology, from Greek Akheron, the name of several real rivers in addition to being the mythical river of the Underworld. The name perhaps means "forming lakes" (compare Greek akherousai "marsh-like water"), from PIE root *eghero- "lake" (source of Lithuanian ežeras, ažeras, Old Prussian assaran, Old Church Slavonic jezero "lake"). The derivation from Greek akhos "woe" is considered folk etymology. The name later was given to certain rivers in Greece and Italy because they flowed through dismal surroundings or disappeared underground and were thought to be gateways to the Underworld. Related: Acherontic.
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.
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.