air-rifle n.

c. 1300, "invisible gases that surround the earth," from Old French air "atmosphere, breeze, weather" (12c.), from Latin aer "air, lower atmosphere, sky," from Greek aēr (genitive aeros) "mist, haze, clouds," later "atmosphere" (perhaps related to aenai "to blow, breathe"), which is of unknown origin. It is possibly from a PIE *awer- and thus related to aeirein "to raise" and arteria "windpipe, artery" (see aorta) on notion of "lifting, suspended, that which rises," but this has phonetic difficulties.

In Homer mostly "thick air, mist;" later "air" as one of the four elements. Words for "air" in Indo-European languages tend to be associated with wind, brightness, sky. In English, air replaced native lyft, luft (see loft (n.)). In old chemistry, air (with a qualifying adjective) was used of any gas.

To be in the air "in general awareness" is from 1875; up in the air "uncertain, doubtful" is from 1752. To build castles in the air "entertain visionary schemes that have no practical foundation" is from 1590s (in 17c. English had airmonger "one preoccupied with visionary projects"). Broadcasting sense (as in on the air, airplay) is by 1927. To give (someone) the air "dismiss" is from 1900. Air pollution is attested by 1870. Air guitar is by 1983. Air traffic controller is from 1956.

"portable firearm having a barrel or barrels with a spirally grooved bore," by 1775; the word was used earlier of the grooves themselves (1751), and is a noun use from rifled pistol, 1680s, from the verb rifle meaning "to cut spiral grooves in" (a gun barrel); see rifle (v.2).

The spirals impart rotation to the projectile, making its flight more accurate. Rifles "troops armed with rifles," sometimes as part of a unit name, is by 1843. Rifle-range is from 1850 as "distance a rifle-ball will carry" (also, and earlier rifle-shot, 1803); the meaning "place for rifle shooting" is by 1862. Rifle-ball is by 1884; the  word continued in use after cylindrical bullets with conical heads replaced round ones.