I looked for about 20 minutes but couldn't find the old ICAO Q Codes. Perhaps you could start with this link from Oz.
I'm not old enough to remember it's common use in aviation, but I have seen an ICAO Convention with a couple of hundred aviation-related Q Codes, such as the QNH you've asked about. Maybe someone else has a good link.
Briefly, they date back to old radio practice, and Morse Code. Surely there are some (other) hams here to go down that road!
Aer Lingus From Ireland, joined May 2000, 1537 posts, RR: 0 Reply 3, posted (13 years 2 months 1 week 5 days 7 hours ago) and read 1192 times:
As far as I can remember, the Q codes as they were known, herald from the war days when it was easier to say QNH, QFE & QNE rather than the full phrase. Even in writing it would be quite a mouthful, just imagine what it would be like on the radio.
Archie From Mexico, joined Aug 2000, 228 posts, RR: 0 Reply 4, posted (13 years 2 months 6 days 23 hours ago) and read 1169 times:
I think the Q codes were used because in the old days they used morse code so it was faster to type QNH in morse code than writing the whole phrase to ask what is the altimeter setting?
That is what I was told once but I don´t know if it is true.
Can anyone confirm this theory?