There are probably others on this forum that can answer this question more thoroughly than I can, but still:
The "Q-codes" were originally a whole bunch of three letter codes, starting with Q. This was sort of a "short hand" from the time when morse codes were used to transmit messages. There were LOTS of Q-codes, which all had a different meaning. QNH being one of them, meaning barometric pressure expressed in hPa, corrected for the altitude of the measuring point.
The Q is to destinguish these special three letter codes from other three letter words, since very few (three letter)words start with Q. (Any at all?) (Yes, Quo.
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The other two letters? -Don't know if there's any system there at all. And that might even be the point.
Other Q-codes: QDM, QDR, QFE, QTE and QPR.
Nick.