It's a British colloquialism - in Britain, the expression "cowboy" is used to describe shoddy workmanship, usually in conjunction with the two most fraudulent and negligent professions over here of builders (incl plumbers and electricians) and car mechanics. It has nothing to do with cowboys of the ranching variety, and absolutely nothing to do with the US, though it may originally stemn from the fact that real cowboys were perceived as being migratory casual workers, which is certainly how many British "cowboy" workers behave - they rarely have fixed working premises as their customers would track them down, which is why they gravitate to the building trade (they botch a job and then move on before the full scale of their bad workmanship even comes to light).
There was nothing in your tone that suggested humour, and a Nonimous' reply got a spiteful answer from you too. You seem to be trying to retrospectively cover an insult as a joke. I'll also wager that you are one of the hypersensitive type who complains the loudest when a genuine joke is made about the US or its inhabitants while insulting just about every other nation.
