Etymology
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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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back-ache (n.)
also backache, "dull or continuous pain in the back," c. 1600, from back (n.) + ache (n.).
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tooth-ache (n.)
also toothache, Old English toðece; see tooth + ache (n.).
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belly-ache (n.)
also bellyache, 1590s, "pain in the bowels," from belly (n.) + ache (n.). The verb in the slang sense of "complain" is first recorded 1888, American English; it appears not to have been used earlier than that, if ever, in a literal sense. Related: bellyached; bellyaching.
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heart-ache (n.)
also heartache, late Old English heort ece "physical pain in or near the heart;" from heart (n.) + ache (n.). Sense of "anguish of mind" is from c. 1600; Old English did, however, have heartsarnes "grief," literally "heart-soreness;" Middle English had herte-smerte "sorrow, contrition."
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earache (n.)

also ear-ache, "pain in the ear," 1789, from ear (n.1) + ache (n.).

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headache (n.)
Old English heafodece; see head (n.) + ache (n.). Colloquial sense of "troublesome problem" is first recorded 1934. Related: Headachy (1705).
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achy (adj.)

"full of aches, sore, aching," 1875, first recorded in George Eliot's letters, from ache (n.) + -y (2). Middle English had akeful "painful" (early 15c.). Related: Achily; achiness.

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