Entries linking to air mail
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) first recorded 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.
"post, letters," c. 1200, "a traveling bag, sack for keeping small articles of personal property," a sense now obsolete, from Old French male "wallet, bag, bundle," from Frankish *malha or some other Germanic source, from Proto-Germanic *malho- (source also of Old High German malaha "wallet, bag," Middle Dutch male "bag"), from PIE *molko- "skin, bag."
The sense was extended to "bag full of letters" (1650s; perhaps via phrases such as a mail of letters, 1654) and "person or vehicle that carries postal matter" (1650s). From thence, to "letters and parcels" generally (1680s) and "the system of transmission by public post" (1690s).
As a newspaper name, by 1789. In 19c. England, mail was letters going abroad, while home dispatches were post. Sense of "a personal batch of letters" is from 1844, originally American English. Mail slot "narrow opening in an exterior door of a building to receive mail delivery" is by 1893, American English. OED defines it as a "letter-slit."