Etymology
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unless (conj.)

mid-15c., earlier onlesse, from (not) on lesse (than) "(not) on a less compelling condition (than);" see less. The first syllable originally was on, but the quality of negation in the word and the lack of stress changed it to un-. "Except could once be used as a synonym for unless, but the words have now drawn entirely apart" [Century Dictionary].

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Anschauung (n.)

"sense-perception," 1833 as a German word in English, nativized from 1848, from German Anschauung "mode of view," literally "a looking at," from anschauen "to look at," from Middle High German aneschouwen, from an (see on) + Old High German scouwon "to look at" (from PIE root *keu- "to see, observe, perceive"). A term in Kantian philosophy.

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neutron (n.)

"electrically neuter particle of the atom," 1921, coined by U.S. chemist William D. Harkins (1873-1951) from neutral (adj.) + -on. First record of neutron bomb, which releases a large number of lethal neutrons but produces little blast, is from 1960. Neutron star attested from 1934, originally hypothetical; so called because it would be composed of densely packed neutrons.

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aloft (adv.)

"on high, in the air," c. 1200, from a Scandinavian source such as Old Norse a lopt "up above," literally "up in the air," from a "in, on" (see on) + lopt "sky, air, atmosphere; loft, upper room," from the general Germanic word for "air" (cognate with Gothic luftus, Old High German luft, Old English lyft "air;" see loft (n.)). Scandinavian -pt- was pronounced as -ft-. The Old English equivalent was on þa lyft.

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lepton (n.)

elementary particle of small mass, 1948, from Greek leptos "small, slight, slender, delicate, subtle," literally "peeled," or "threshed out" (from lepein "to peel," from PIE *lep- (1), a root which yields words for "peel" as well as "small shaving, scale (of a fish)," hence used of things fine, delicate, or weak; see leper) + -on. In Greek it was the name of a small coin, from neuter of leptos. Related: Leptonic.

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anvil (n.)

Old English anfilt "anvil," a Proto-Germanic compound (source also of Middle Dutch anvilt, Old High German anafalz, Dutch aanbeeld, Danish ambolt "anvil"), apparently representing *ana- "on" (see on (prep.)) + *filtan "to hit" (from PIE root *pel- (5) "to thrust, strike, drive").

The ear bone (incus) is so called from 1680s. The musical Anvil Chorus is based on the "Gypsy Song" that opens Act II of Verdi's "Il Trovatore," first performed in Teatro Apollo, Rome, Jan. 19, 1853.

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upon (prep.)

early 12c., from Old English uppan (prep.) "on, upon, up to, against," from up (adv.) + on (prep.); probably influenced by Scandinavian sources such as Old Norse upp a.

On, Upon. These words are in many uses identical in force, but upon is by origin (up + on) and in use more distinctly expressive of motion to the object from above or from the side. On has the same force, but is so widely used in other ways, and so often expresses mere rest, that it is felt by careful writers to be inadequate to the uses for which upon is preferred. [Century Dictionary, 1895]
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acknowledge (v.)

late 15c., "admit or show one's knowledge," a blend of Middle English aknow "admit or show one's knowledge" and Middle English knowlechen "admit, acknowledge" (c. 1200; see knowledge). Middle English aknow is from Old English oncnawan "understand, come to recognize," from on (see on (prep.)) + cnawan "recognize;" see know).

"By 16th c. the earlier vbs. knowledge and a(c)know ... were obs., and acknowledge took their place" [OED]. In the merger, an unetymological -c- slipped in; perhaps the explanation is that when English kn- became a simple "n" sound, the -c- stepped up to preserve, in this word, the ancient "kn-" sound. Related: Acknowledged; acknowledging.

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in (adv., prep.)

a Middle English merger of Old English in (prep.) "in, into, upon, on, at, among; about, during;" and Old English inne (adv.) "within, inside," from Proto-Germanic *in (source also of Old Frisian, Dutch, German, Gothic in, Old Norse i), from PIE root *en "in." The simpler form took on both senses in Middle English.

Sense distinction between in and on is from later Middle English, and nuances in use of in and at still distinguish British and American English (in school/at school). Sometimes in Middle English shortened to i.

The noun sense of "influence, access (to power or authorities)," as in have an in with, is first recorded 1929 in American English. to be in for it "certain to meet with something unpleasant" is from 1690s. To be in with "on friendly terms with" is from 1670s. Ins and outs "intricacies, complications of an action or course" is from 1660s. In-and-out (n.) "copulation" is attested from 1610s.

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again (prep., adv.)

late Old English agan, from earlier ongean (prep.) "toward; opposite, against, contrary to; in exchange for," as an adverb "in the opposite direction, back, to or toward a former place or position," from on "on" (see on (prep.) and compare a- (1)) + -gegn "against, toward." This is from the Germanic root *gagina (source also of Old Norse gegn "straight, direct;" Danish igen "against;" Old Frisian jen, Old High German gegin, German gegen "against, toward," entgegen "against, in opposition to").

In Old English, eft (see eftsoons) was the main word for "again," but this often was strengthened by ongean, which became the principal word by 13c. Norse influence is responsible for the hard -g-. It was differentiated from against (q.v.) 16c. in southern writers, again becoming an adverb only, and against taking over as preposition and conjunction, but again clung to all senses in northern and Scottish dialect (where against was not adopted). Of action, "in return," early 13c.; of action or fact, "once more," late 14c.

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