In E.E. "Doc" Smith's Lensman series, "QX" is used by the Galactic Patrol to mean "OK", "all right", "fine", "roger", and sometimes even "yes, sir".
The origins of the term are unclear - one possibility is simply that "QX" is shaped somewhat like "OK".
Many words, now common and familiar in English, were originally introduced in science fiction. A few that come to mind immediately are grok, warp speed, borg, cyborg, android, robot, gadget, clone, hyperspace, and gubbish. Even some one-word epithets like "Blast!" have insinuated themselves into "normal" speech. Now, "QX" is extremely easy to pick up when reading the books - but so far it hasn't become a part of this lexicon.