Etymology
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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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air mail (n.)

also air-mail, airmail, 1913, from air (n.1) meaning "by aircraft" + mail (n.1). As a verb by 1919. Related: Air-mailed.

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air force (n.)
1917, from air (n.1) + force (n.); first attested with creation of the Royal Air Force. There was no United States Air Force until after World War II. The Air Corps was an arm of the U.S. Army. In 1942, the War Department reorganized it and renamed it Army Air Forces. The National Security Act of 1947 created the Department of the Air Force, headed by a Secretary of the Air Force, and the U.S.A.F.
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Cloud Cuckoo Land 

imaginary city built in air, 1830, translating Aristophanes' Nephelokokkygia in "The Birds" (414 B.C.E.). Cloud-land "place above the earth or away from the practical things of life, dreamland, the realm of fancy" is attested from 1840.

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science fiction (n.)

1929 (in advertisements for "Air Wonder Stories" magazine), though there is an isolated use from 1851. See science + fiction. Earlier in same sense was scientifiction (by 1926). Abbreviated form sci-fi is by 1955. 

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al fresco (adv.)
also alfresco, 1753, Italian, literally "in the fresh (air)." Italian al represents a contraction of words from Latin ad "to" (see ad-) + ille "that" (see le). Alfresco also meant "painted on plaster that was still fresh or moist" (1764; see fresco).
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bug off (v.)
"leave quickly," by 1956, perhaps from bugger off (see bugger (v.)), which chiefly is British (by 1920s) but was picked up in U.S. Air Force slang in the Korean War. Also see bug (v.3). To bug out "leave quickly, scram" is from 1953.
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carbon dioxide (n.)
1869, so called because it consists of one carbon and two oxygen atoms. The chemical was known since mid-18c. under the name fixed air; later as carbonic acid gas (1791). "The term dioxide for an oxide containing two atoms of oxygen came into use in the middle of the 19th century." [Flood].
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middle passage (n.)

"part of the Atlantic Ocean which lies between the West Indies and the west coast of Africa," 1788, in the agitation against the trans-Atlantic slave trade, from middle (adj.) + passage.

It is clear that none of the unfortunate people, perhaps at this moment on board, can stand upright, but that they must sit down, and contract their limbs within the limits of little more than three square feet, during the whole of the middle passage. I cannot compare the scene on board this vessel, to any other than that of a pen of sheep; with this difference only, that the one have the advantages of a wholesome air, while that, which the others breathe, is putrid. [Thomas Clarkson, "An Essay on the Slavery and Commerce of the Human Species," 1788]
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