The question is similar to the question of what BNC or DVD stands for.
Most answers look for what the letters could stand for rather than how
they originated. I'm only interested in what inspired the choice in
the first place, and that is not always easy to determine.

The meaning of XLR may never be known, unless Cannon was thoughtful
about documenting it; we may have to settle for a plausible
explanation and hope for the best. If the designer of the connector
is still alive, he might be able to help.

In the case of DVD, we're fortunate. Most of us were around when the
acronym was invented and know the history. DVD stands for "Digital
Video Disc." Digital Versatile Disc was simply a revision applied
when the disc began to be used for other purposes. I suspect that
most definitions of XLR fall into the same category (as do the choices
for BNC.)

We may never know for sure.

Norm Strong