Etymology
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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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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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-head 

word-forming element meaning "state or condition of being," Middle English -hede, from a variant of Old English -had, the source of -hood. The only surviving words with it are maidenhead and godhead.

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head (v.)

"to be at the head or in the lead," c. 1200, from head (n.). Meaning "to direct the head (toward)" is from c. 1600. Related: headed, heading. The earliest use of the word as a verb meant "behead" (Old English heafdian). Verbal phrase head up "supervise, direct" is attested by 1930.

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head (adj.)

"most important, principal, leading," c. 1200, from head (n.). Old English heafod was used in this sense in compounds.

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head (n.)

Old English heafod "top of the body," also "upper end of a slope," also "chief person, leader, ruler; capital city," from Proto-Germanic *haubid (source also of Old Saxon hobid, Old Norse hofuð, Old Frisian haved, Middle Dutch hovet, Dutch hoofd, Old High German houbit, German Haupt, Gothic haubiþ "head"), from PIE root *kaput- "head."

Modern spelling is early 15c., representing what was then a long vowel (as in heat) and remained after pronunciation shifted. Of rounded tops of plants from late 14c. Meaning "origin of a river" is mid-14c. Meaning "obverse of a coin" (the side with the portrait) is from 1680s; meaning "foam on a mug of beer" is first attested 1540s; meaning "toilet" is from 1748, based on location of crew toilet in the bow (or head) of a ship.

Synechdochic use for "person" (as in head count) is first attested late 13c.; of cattle, etc., in this sense from 1510s. As a height measure of persons, from c. 1300. Meaning "drug addict" (usually in a compound with the preferred drug as the first element) is from 1911.

To be over (one's) head "beyond one's comprehension" is by 1620s. To give head "perform fellatio" is from 1950s. Phrase heads will roll "people will be punished" (1930) translates Adolf Hitler. Head case "eccentric or insane person" is from 1966. Head game "mental manipulation" attested by 1972.

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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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back-ache (n.)

also backache, "dull or continuous pain in the back," c. 1600, from back (n.) + ache (n.).

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cat-head (n.)

"beam projecting from each side of the bows of a ship to hold the anchor away from the body of the ship," 1620s, from cat (n.) in some obscure sense + head (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 recorded by 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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