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poacher turned gamekeeper
A person whose job or role has completely changed, from one extreme to another. Primarily heard in UK. The notorious computer hacker who once stole money from unsuspecting Internet users now helps the government catch other hackers—it's a strange case of poacher turned gamekeeper.
old poacher makes the best gamekeeper
Prov. The best person to guard something is someone who knows all about how to steal it, so he or she can anticipate what thieves might do. We should hire the computer hacker to design computer security systems. An old poacher makes the best gamekeeper.
poacher turned gamekeeperBRITISH
If you describe someone as poacher turned gamekeeper, you mean that they have changed their job or opinion and now have authority over the same sort of people that they were themselves before. She returns as poacher turned gamekeeper. Previously a union leader, she now works for the Committee of Vice-Chancellors and Principals. Gary Mason, boxing's poacher turned gamekeeper, will make his managerial debut tomorrow. Note: You can also say that someone is gamekeeper turned poacher, when someone who was in authority changes to a situation where they need to be controlled by others. He used to manage properties for other landlords. Now the gamekeeper turned poacher has to find occupiers for his own building. Note: A poacher is someone who illegally hunts animals such as deer, pheasant or salmon. A gamekeeper is someone employed by a landowner to look after these animals, which are known as game.