“You're a criminal mastermind.”
You're talking with a friend about hotels. Your friend tells you that he sometimes steals the towels when he stays in a hotel. You make fun of him for stealing something that's not very valuable.
You're a criminal mastermind.
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A "mastermind" is someone who controls people and events by being very, very smart. Sometimes being "a mastermind" is positive:
She's a mastermind when it comes to sales and marketing!
Sometimes it's very negative:
Police have arrested the mastermind behind the recent terrorist attacks.
A "criminal mastermind" is someone who leads a criminal organization like the mafia, or someone who commits crimes that take a lot of planning.
You can also use the phrase "criminal mastermind" sarcastically or jokingly to talk about a criminal who's not very smart:
He tried to steal money from the cash register at work but didn't think about the camera pointed straight at him. Not exactly a criminal mastermind!