a criminal mastermind

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!

This phrase appears in these lessons: