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!