I have no idea how today's word came to mean something so different across the Pond!
"We managed to employ an absolute Cowboy builder on our house"
'Cowboy' = Someone useless who makes a total mess of the work
This term is used most commonly to describe tradesmen you might employ like Carpenters, Electricians, Builders. It is a phrase you will hear regularly, there are even documentary TV shows with this phrase in their title.
Cowboy if anything intimates the exact opposite here (someone who works hard outdoors) and it is merely a description of an actual person who is working on a ranch.