this is informative:

http://www.strategyand.pwc.com/global/home/cookie-information

Quote:
The "__cfduid" cookie is set by the CloudFlare service to identify trusted web traffic. It does not correspond to any user id in the web application, nor does the cookie store any personally identifiable information.
https://support.cloudflare.com/hc/en-us/...duid-cookie-do-

It doesn't appear to be a multipurpose or "common name to use" cookie, it just has one purpose, and is used heavily for that purpose. (and with a five year persistence)

the "cf" is CouldFlare, not sure on the "d", and "uid" in this case is Unique IDentifier, not "User ID". (I see "UID" used equally in both contexts around computers)

I work for the Department of Redundancy Department