Etymology
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air-lock (n.)

by 1851, "air-tight chamber in which operations are carried on under water," to regulate pressure for the safety of workers, from air (n.1) + lock (n.1) in the canal sense.

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mid-air (n.)

also midair, "the part of the air between the clouds and the air near the ground," from mid (adj.) + air (n.1).

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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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put-on (n.)

"ruse, deception," 1937, from earlier adjectival meaning "assumed, feigned" (1620s), a figurative extension of the verbal phrase on the notion of putting on costumes or disguises. To put on (v.), of clothes, garments, etc., is by early 15c.; see from put (v.) + on (adv.). Hence "clothe, cover, assume as covering" (mid-15c.) and "assume the garb or appearance of" (real or feigned), 1520s. The expression put (someone) on "play a trick on, deceive" (by 1958) seems to be a back-formation from the noun.

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air-hole (n.)

"an opening to admit or discharge air," 1766, from air (n.1) + hole (n.).

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open-air (adj.)

"outdoor, in the open air," 1520s, from open (adj.) + air (n.1).

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turn-on (n.)
that which arouses or excites, 1968, originally of psychedelic drugs, from verbal phrase turn on "activate (a mechanism)" (1833), specifically from figurative sense turn (someone) on "excite, stimulate, arouse," recorded from 1903; from turn (v.) + on (adv.).
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iron-on (adj.)
1959, from the verbal phrase, from iron (v.) + on (adv.).
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