Etymology
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onset (n.)

1530s, "attack, assault, a rushing or setting upon," from on + set (n.); compare verbal phrase to set (something) on (someone), c. 1300, originally "sic (a dog) on." Weaker sense of "beginning, start" is recorded from 1560s. Figurative use in reference to a calamity, disease, etc. is from 1580s. Middle English had set (n.3) "attack, onslaught" (mid-14c.).

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onto (prep.)

"toward and upon; to and in connection with; to the top of," 1580s, on to, from on + to. It appeared much later than parallel into, unto. As a closed compound, onto (on analogy of into), it is recorded from 1715. "The word is regarded by purists as vulgar, and is avoided by careful writers" [Century Dictionary, 1895].

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hadron (n.)
1962, from Greek hadros "thick, bulky" (the primary sense), also "strong, great; large, well-grown, ripe," from PIE root *sa- "to satisfy." With elementary particle suffix -on. Coined in Russian as adron.
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anlage (n.)
"basis of a later development" (plural anlagen), 1892, from German anlage "foundation, basis," from anlagen (v.) "to establish," from an "on" (see on (prep.)) + legen "to lay" (from PIE root *legh- "to lie down, lay").
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a (2)
as in twice a day, etc., a reduced form of Old English an "on" (see on (prep.)), in this case "on each." The sense was extended from time to measure, price, place, etc. The habit of tacking a onto a gerund (as in a-hunting we will go) was archaic after 18c.
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onslaught (n.)

"attack, aggression, assault," 1620s, anslaight, apparently somehow from or on analogy of Dutch aanslag "attack," from Middle Dutch aenslach, from aen "on" (see on) + slach "blow," related to slaen "slay." Early sources say the word was from Dutch, but the forms do not correspond well. The spelling would have been influenced by English slaught (n.) "slaughter," from Old English sleaht (see slaughter (n.)), but this word, obsolete since c. 1400, can't be the source of the modern one unless the record is imperfect. There is no record of onslaught in 18c.; apparently the word was revived by Scott.

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afar (adv.)

"from far, from a distance," a contraction of Middle English of feor (late 12c.), on ferr (c. 1300), from Old English feor "far" (see far); the a- (1) in compounds representing both of and on (which in this use meant the same). Spelled afer in 14c.

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ana- 

before vowels an-, word-forming element meaning: 1. "upward, up in place or time," 2. "back, backward, against," 3. "again, anew," from Greek ana (prep.) "up, on, upon; up to, toward; throughout; back, backwards; again, anew," from an extended form of PIE root *an- (1) "on, upon, above" (see on, which is the English cognate). In old medical prescriptions, ana by itself meant "an equal quantity of each."

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anschluss (n.)
1924 as a German word in English, from German Anschluß, "connection; addition; junction," literally "joining, union," from anschließen "to join, annex," from an "at, to, toward" (from Old High German ana- "on;" see on) + schließen "to shut, close, lock, bolt; contract" (a marriage); see slot (n.2). Specifically the Pan-Germanic proposal to unite Germany and Austria, accomplished in 1938.
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meson (n.)

subatomic particle, 1939, from Greek mesos "middle" (from PIE root *medhyo- "middle") + subatomic particle suffix -on. Earlier mesotron (1938). So called for being intermediate in mass between protons and electrons. An earlier use of the word, from the Greek noun meson "center," meant "the medial plane which divides the body into two equal and symmetrical parts" (by 1883).

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