Etymology
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on (prep., adv.)

"in a position above and in contact with; in such a position as to be supported by;" also noting the goal to which some action is or has been directed; "about, concerning, regarding; in a position to cover;" as an adverb, "in or into a position in contact with and supported by the top or upper part of something; in or into place; in place for use or action; into movement or action; in operation," Old English on, unstressed variant of an "in, on, into," from Proto-Germanic *ana "on" (source also of Dutch aan, German an, Gothic ana "on, upon"), from PIE root *an- (1) "on" (source also of Avestan ana "on," Greek ana "on, upon," Latin an-, Old Church Slavonic na, Lithuanian nuo "down from").

Also used in Old English in many places where we now would use in. From 16c.-18c. (and still in northern England dialect) often reduced to o'. Phrase on to "aware" is from 1877.

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on- 

the preposition and adverb on used as a prefix; Old English on-, an-.

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clip-on (adj.)

"held on by means of a clip," 1909, from the verbal phrase; see clip (v.2) + on (adv.).

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hard-on (n.)
"penile erection," 1922, earlier as an adjective (1893), from hard + on.
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add-on (n.)
"additional component," 1941, from verbal phrase add on; see add (v.) + on (adv.).
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hang on (v.)
1860, "to remain clinging," 1860, especially "cling fondly to" (1871); see hang (v.) + on (adv.). As a command to be patient, wait a minute, from 1936, originally in telephone conversations.
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iron-on (adj.)
1959, from the verbal phrase, from iron (v.) + on (adv.).
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on-site (adj.)
also onsite, 1959, from on + site. Originally in reference to Cold War military inspections.
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off-and-on (adv.)

"intermittently, occasionally," 1530s; see off (adv.) + on. As an adjective, "occasional," from 1580s.

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