Etymology
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head over heels (adv.)

1726, "a curious perversion" [Weekley] of Middle English heels over head (late 14c.) "somersault fashion," hence "recklessly." Head (n.) and heels long have been paired in alliterative phrases in English, and the whole image also was in classical Latin (per caput pedesque ire).

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Good Friday (n.)

the Friday before Easter, c. 1300, from good (adj.) in Middle English sense of "holy, sacred," especially of holy days or seasons observed by the church; the word also was applied to Christmas and Shrove Tuesday. Good Twelfthe Dai (c. 1500) was Epiphany (the twelfth day after Christmas).

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pea jacket (n.)

"heavy coat generally worn by sailors in cold or stormy weather," 1721, a partial loan-translation of North Frisian pijekkat, from Dutch pijjekker, from pij "coarse woolen cloth" + jekker "jacket." Middle English had pee "coat of coarse, thick wool" (late 15c.). Related: Pea-coat.

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in medias res 

Latin, literally "in the midst of things," from medias, accusative fem. plural of medius "middle" (see medial (adj.)) + accusative plural of res "a thing" (see re). From Horace, in reference to narrative technique:

Semper ad eventum festinat, et in medias res,
Non secus ac notas auditorem rapit (etc.)
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Sieg Heil (interj.)

Nazi salute, German, literally "hail victory;" from German Sieg "victory," from Old High German sigu (see Siegfried) + heil "to hail," from Proto-Germanic *hailitho (see health). English heil was used in Middle English as a salutation implying respect or reverence (c. 1200; see hail (interj.)).

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carbon dioxide (n.)

1869, so called because it consists of one carbon and two oxygen atoms. The chemical was known since mid-18c. under the name fixed air; later as carbonic acid gas (1791). "The term dioxide for an oxide containing two atoms of oxygen came into use in the middle of the 19th century." [Flood].

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Santa Claus (n.)

1773 (as St. A Claus, in "New York Gazette"), American English, in reference to the customs of the old Dutch colony of New York, from dialectal Dutch Sante Klaas, from Middle Dutch Sinter Niklaas "Saint Nicholas," bishop of Asia Minor who became a patron saint for children. Now a worldwide phenomenon (Japanese santakurosu). Father Christmas is attested from 1650s.

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court-martial (n.)

also court martial, "court of military or naval officers to try cases of desertion, mutiny, etc.," 1650s (plural courts martial), originally martial court (1570s), from court (n.) + martial (adj.). Word-order changed on the model of French cour martiale. As a verb, from 1859. Related: Court-martialed. Middle English had court-spiritual "ecclesiastical court" (late 15c.).  

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humble pie (n.)

to eat humble pie (1830) is from umble pie (1640s), pie made from umbles "edible inner parts of an animal" (especially deer), considered a low-class food. The similar sense of similar-sounding words (the "h" of humble (adj.) was not then pronounced) converged to make the pun. Umbles is Middle English numbles "offal," with loss of n- through assimilation into preceding article.

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coat of arms (n.)

mid-14c., also simply coat (mid-14c.); originally a tunic embroidered or painted with heraldic armorial bearings (worn over armor, etc); see coat (n.) + arm (n.2) and compare Old French cote a armer. Sense transferred in Middle English to the heraldic arms themselves. Hence turncoat, one who put his coat on inside-out to hide the badge of his loyalty (1550s).

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