The 'x' suffix is aviation speak for an abbreviated word called a [contraction][1].  Some are defined in manuals or advisory circulars.  For example, WX is weather as defined in an old advisory circular, AC00-45.

Official or not, a few others that come to mind are:

    MX - maintenance
    CX - cancel
    TX - transmit
    RX - receive

Maybe others will chime in the comments with more lore.


  [1]: https://aviationweather.gov/static/docs/awc-contractions-contsort.pdf