Etymology
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Jack-o'-lantern (n.)

also jack-o-lantern, jack-a-lantern, jackolantern, 1660s, "night-watchman;" 1670s as a local name for a will-o-the-wisp (Latin ignis fatuus), mainly attested in East Anglia but also in southwestern England. Literally "Jack of (with) the lantern;" see Jack + lantern. The extension to carved pumpkin lanterns is attested by 1834 in American English.

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will-o'-the-wisp (n.)

1660s, earlier Will with the wisp (c. 1600), from the masc. proper name Will + wisp "bundle of hay or straw used as a torch." Compare Jack-o'-lantern.

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lantern (n.)

mid-13c., from Old French lanterne "lamp, lantern, light" (12c.), from Latin lanterna "lantern, lamp, torch," altered (by influence of Latin lucerna "lamp") from Greek lampter "torch, beacon fire," from lampein "to shine, give light, be brilliant" (from PIE root *lap- "to light, burn;" see lamp).

Variant lanthorn (16c.-19c.) was folk etymology based on the common use of horn as a translucent cover. Lantern-jaws "hollow, long cheeks" is from a resemblance noted at least since mid-14c.; Johnson suggests the idea is "a thin visage, such as if a candle were burning in the mouth might transmit the light."

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O 

fifteenth letter of the alphabet, from a character that in Phoenician was called  'ain (literally "eye") and represented "a very peculiar and to us unpronounceable guttural" [Century Dictionary]. The Greeks also lacked the sound, so when they adopted the Phoenician letters they arbitrarily changed O's value to a vowel. (Thus there is no grounds for the belief that the form of the letter represents the shape of the mouth in pronouncing it.) The Greeks later added a special character for "long" O (omega), and the original became "little o" (omicron).

In Middle English and later colloquial use, o or o' can be an abbreviation of on or of, and is still literary in some words (o'clock, Jack-o'-lantern, tam-o'-shanter, cat-o'-nine-tails, will-o'-the-wisp, etc.).

O' the common prefix in Irish surnames is from Irish ó, ua (Old Irish au, ui) "descendant." 

The "connective" -o- is the usual connecting vowel in compounds taken or formed from Greek, where it often is the vowel in the stem. "[I]t is affixed, not only to terms of Greek origin, but also to those derived from Latin (Latin compounds of which would have been formed with the L. connecting or reduced thematic vowel, -i), especially when compounds are wanted with a sense that Latin composition, even if possible, would not warrant, but which would be authorized by the principles of Greek composition." [OED]

As "zero" in Arabic numerals it is attested from c. 1600, from the similarity of shape. Similarly the O blood type (1926) was originally "zero," denoting the absence of A and B agglutinogens.

As a gauge of track in model railroads, by 1905. For o as an interjection of fear, surprise, joy, etc., see oh.

The use of the colloquial or slang -o suffix in wino, ammo, combo, kiddo, the names of the Marx Brothers, etc., "is widespread in English-speaking countries but nowhere more so than in Australia" [OED].

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Jack 

masc. proper name, attested by 1218, probably via Anglo-French Jake, Jaikes, from Old French Jacques (which was a diminutive of Latin Jacobus; see Jacob), but in English the name always has been regarded as a familiar form of John, and some have argued that it is a native formation. In Middle English spelled Jakke, Jacke, etc., and pronounced as two syllables ("Jackie").

In England, Jack became a generic name applied familiarly or contemptuously to anybody (especially a young man of the lower classes) from late 14c. Later used especially of sailors (1650s; Jack-tar is from 1781); Jack-ashore (adj.) "drinking and in high spirits, recklessly spending" (1875) also is an image from sailors (1840 as a book title). In U.S., as a generic name addressed to an unknown stranger, attested from 1889. Every man Jack "everyone" is from 1812. Also see jack (n.).

Used in male personifications from 15c.; first record of jack-of-all-trades "person handy at any kind of work or business" is from 1610s; Jack Frost is from 1826; Jack-nasty "a sneak or sloven" is from 1833 (Jack-nasty-face, a sea-term for a common sailor, is from 1788). Jack Sprat for a small, light man is from 1560s (his opposite was Jack Weight). Jack-pudding "comical clown, buffoon" is from 1640s. Jack-Spaniard is from 1703 as a Spaniard, 1833 as "a hornet" in the West Indies. Other personifications listed in Farmer & Henley include jack-snip "a botching tailor," Jack-in-office "overbearing petty official" (1680s), Jack-on-both-sides "a neutral," Jack-out-of-doors "a vagrant" (1630s), jack-sauce "impudent fellow" (1590s).

The U.S. plant jack-in-the-pulpit (Indian turnip) is attested by 1833. Jack the Ripper was active in London 1888. The Scottish form is Jock (compare jockey (n.)). Alliterative coupling of Jack and Jill is from 15c. (Iakke and Gylle, Ienken and Iulyan). Jack Ketch for "hangman, executioner" (1670s) is said to be from the name of a public executioner in the time of James II (compare Derrick); it also was used as a verb meaning "to hang."

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jack (n.)

late 14c., jakke "a mechanical device," from the masc. name Jack. The proper name was used in Middle English for "any common fellow," and thereafter extended to various appliances which do the work of common servants (1570s). Also used generically of male animals (1620s, see jackass, jackdaw, etc.).

As a portable contrivance for raising weight by force from below, 1703. As the name of a device for pulling off boots from 1670s. The jack in a pack of playing cards (1670s) is in German Bauer "peasant." Slang meaning "money" is by 1890 (in earlier slang it meant "a small coin"). Jack-towel, one sewn together at the ends round a roller, is from 1795. The jack of Union Jack is a nautical term for "small flag at the bow of a ship" (1630s) and perhaps is from the word's secondary sense of "smaller than normal size."

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jack (v.)

1860, jack up "hoist, raise, lift with a jack," American English, from jack (n.) in the appliance sense. Figurative sense "increase (prices, etc.)" is 1904, American English. Related: Jacked; jacking. Jack off (v.) "masturbate" is attested from 1916, probably from jack (n.) in the old slang sense of "(erect) penis."

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jack-rabbit (n.)

also jackrabbit, large prairie hare, 1863, American English, shortening of jackass-rabbit (1851; see jackass + rabbit (n.)); so called for its long ears. Proverbial for bursts of speed (up to 45 mph).

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jack-knife (n.)

also jackknife, "pocket knife larger than a pen-knife," 1711, probably American English, apparently from some sense of jack (n.). Perhaps it originally was associated with sailors. Jackleg, jacklegged was a U.S. colloquial term of contempt from 1839. Scottish dialect had jockteleg (1670s) "large clasp-knife," of unknown origin, also jackylegs, jack-o-legs. As a kind of swimming dive from 1922; as a type of tractor-trailer accident, 1966; both from the notion of folding, as the knife does.

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Jell-O (n.)

also sometimes Jello, trademark for powdered gelatin food, advertised from 1900 by Genesee Pure Food Co., Le Roy, N.Y.

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