What is the origin of the term "pax"?
The 'x' suffix is aviation speak in an abbreviated word called a contraction.  You can think of the 'x' as &quot;cut short&quot; shorthand.  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. The -x is also used outside of aviation, and is a common abbreviation marker, e.g. Dx (&quot;diagnosis&quot;), elex (&quot;election&quot;), and Hx (&quot;history&quot;).