Etymology
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air-rifle (n.)
rifle that uses compressed air power to fire the projectile, 1851, from air (n.1) + rifle (n.).
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air-space (n.)

also airspace, by 1821 in reference to stove and furnace construction, from air (n.1) + space (n.). From 1852 in reference to the cubic contents of a room (with reference to the persons in it) in sanitary regulations for boarding rooms, hospitals, etc. In firearms, "a vacant space between the powder charge and the projectile" (1847). By 1910 as "portion of the atmosphere controlled by a country above its territory."

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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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hot air (n.)
"unsubstantiated statements, boastful talk," 1900, from hot (adj.) + air (n.1). The adjectival phrase hot-air (of balloons, etc.) is from 1813.
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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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