Do you know what a cowboy is?
Yes, of course you do, just like in the movies.
So what about a cowboy builder? Bob the builder with a cowboy hat maybe? Well, maybe but more often it means a dishonest one. It's someone who does work that doesn't need doing, does it badly and overcharges. (Could also be a management consultant!)
Here's a cowboy builder caught on camera in the BBC series Rogue Traders. It gets quite heated towards the end; almost a battle of cowboys and Indians!