People on here and on other networks often use CDN to refer to Canada, which I have found confusing. People use it for e.g. hashtags like #'cdnpol or #'cdnmedia .
I finally looked it up and according to a random commenter on a page[0] on The Guardian's website and according to en.wikipedia.org/wiki/Internat… (marked "citation needed") it's for the Canadian DominioN.
In the Olympics, Canada is CAN.
en.wikipedia.org/wiki/List_of_…
The ISO alpha-3 code is also CAN.
en.wikipedia.org/wiki/ISO_3166…
So it seems it's really just the car sticker thing that got this peculiar abbreviation. And now social media.