GW Bush was from New Haven, Connecticut and people called him a "cowboy'', or referred to his policies as "cowboy politics", because they saw him as reckless and lacking in thought.
The term "cowboy" is an old one and the English version originally just referred to someone (usually male) who literally took care of cows. It had a slightly more detailed meaning in Spanish and usually indicated someone who rode a horse and
took care of cows. By the 1880's, in the US, it was often used as term for a male who was a troublemaker or who broke the law; probably because the western cowboys had a tendency to drink, womanize and get into trouble after long weeks on the road with nothing to do but escort cattle. I think today it can have the old literal meaning, romantic meaning (the sexy, loner like Clint Eastwood) or it can be used as a slander to accuse someone of who is seen as being out of control.