What does the CO in "CO.UK domain names" mean?
 

CO.IL, CO.JP and many others.

First, i thought it was "commonwealth" but i don't think Japan belongs to it :)

Company?

Commercial?
One of those.

co.ck, stands for cock. Dont know about co.uk though.

it's commercial, the same as .com but is country specific

co.uk -> commercial
org.uk -> non profit organizations
gov.uk -> government

etc.

i always wandered why in some countries like UK or Israel for example the most popular TLD is in the form of .co.uk or .co.il
but in others, like russia, it's simply .ru
does .co.ru even exist? I never saw any site that ends with .co.ru extension

copulation of course..

I thought it was country... as in .co(untry).uk

It's commercial:

co.uk - commercial/general