If you search rec.audio.pro (always a good idea)
you will find this post by Jeremy Shaw:

Cannon has a range of connectors in the XL series, IE: XLA, XLB, and
of course, XLR.
You will see that in most catalogs, connectors manufactured by other
companies are called "XLR-type" connectors, so, just like Kleenex has
become a generic (although still trademarked) term for facial tissue,
XLR has become the the term in general usage for this type of
connector.
Now that all that has been said, I have also heard the explanation
that "X" refers to the ground, and "L" and "R" refer to left and right
(or line and return). While this makes a neat explanation for the
XLR-3, it does not explain the XLR-4,5,6, or 7.

In reality XLR is just a part number assigned by ITT/Cannon

X for the series the Military used (or just another letter in their
commercial series of connectors after U and UA)
L for locking
R for rubber

The XLR connector was designed for Boeing in the 60's.
by Cannon (and there was an XL connector before, with a rigid
insulator)
(also I had not heard that it was for Boeing, but could believe that
.... it was commonly used as an inexpensive instrumentation connector
when people wanted something that would lock
but not have a threaded ring)

Then Switchcraft and others used it for audio.
Switchcraft added a grounding lug to connect to the shell, and went
back
to a rigid insulator, which was a bad idea -- the rubber female side
was
tensioned so there was a good strong wiping action to get rid of the
contact oxidation at every mating cycle

In the 80'sit became commonly available with gold
plated pins instead of the silver plating on the old ones that
commonly
became silver oxide and other less conductive silver salts... so that
the scraping/wiping action made by the original XLR wasn't needed...

Note, that ITT/Cannon is different than Canon the copier company.

It is also interesting to note that ITT/Cannon no longer manufactures
the Cannon XLR connectors (or so I have heard). I believe they sold
the patent to Switchcraft.

Jeremy Shaw.