Etymology
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isthmus (n.)
1550s, from Latin isthmus, from Greek isthmos "narrow passage, narrow neck of land between two seas," originally especially that of Corinth, a word of unknown origin. Perhaps from eimi "to go" + suffix -thmo (compare ithma "a step, movement").
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isthmian (adj.)
c. 1600, from Latin isthmius, from Greek isthmios "pertaining to the isthmus (of Corinth)," from isthmos (see isthmus). Originally in reference to the Isthmian Games, held at Corinth in honor of Poseidon.
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peninsula (n.)

"piece of land almost surrounded by water but connected with a mainland by a neck or isthmus," 1530s, from Latin paeninsula "a peninsula," literally "almost an island," from pæne "nearly, almost, practically," which is of uncertain origin, + insula "island" (see isle). In 16c. sometimes Englished as demie island.

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neck (n.)

"that part of an animal body between the head and the trunk and which connects those parts," Middle English nekke, from Old English hnecca "neck, nape, back of the neck" (a fairly rare word) from Proto-Germanic *hnekk- "the nape of the neck" (source also of Old Frisian hnekka, Middle Dutch necke, Dutch nek, Old Norse hnakkr, Old High German hnach, German Nacken "neck"), with no certain cognates outside Germanic, though Klein's sources suggest PIE *knok- "high point, ridge" (source of Old Irish cnocc, Welsh cnwch, Old Breton cnoch "hill").

The more usual Old English words were hals (the general Germanic word, cognate with Gothic, Old Norse, Danish, Swedish, Dutch, German hals), from Proto-Germanic *halsaz, which is perhaps cognate with Latin collum (see collar (n.)); and sweora, swira "neck, nape," probably also from a PIE root meaning "column" (cognate with Old English swer "column," Sanskrit svaru- "post").

Oxen and other draught animals being yoked by the neck, it became a symbol of burdens, of submission or subjugation, and also resistance or obstinacy (compare stiff-necked). Figuratively, "life" (late 15c.) from the breaking or severing of the neck in legal executions. Meaning "narrow part at the top of a bottle" is from late 14c.; meaning "part of a garment which covers the neck" is from 1520s. Meaning "long, slender part of a stringed musical instrument" is from 1610s.

Sense of "isthmus, long, narrow strip of land connecting two larger ones" is from 1550s. Phrase neck of the woods (American English) is attested from 1780 in the sense of "narrow stretch of woods;" 1839 with meaning "settlement in a wooded region." To stick (one's) neck out "take a risk" is recorded by 1919, American English. Horses running neck and neck "at an equal pace" is attested from 1799; to win by a neck is from 1823. To be up to the neck "have a lot of" at first (mid-19c.) suggested "fed full," but since c. 1900 it has implied "in deep."

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