What it means
A mastermind is the brilliant brain behind a complex plan, the one who works out every angle and pulls the strings while everyone else just follows the steps. It's the criminal genius plotting the heist, but also the friend who orchestrates the surprise party down to the minute. Equal parts admiration and a slightly nervous what-are-they-up-to.
Usage examples
"She was the mastermind behind the whole campaign, every detail was her idea."
"They never caught the mastermind, only the small fry who did the legwork."
"Don't look at me, Priya was the mastermind who mapped the whole thing out on the group chat before breakfast."
"He's not just late to everything, he's the mastermind of every dodgy little office prank going."
Where it comes from
Mastermind is a straight compound of master and mind. It showed up in English in the late 19th century for the person supplying the brains behind a plan or operation, then really sank its teeth into crime fiction, detective stories, and headlines about the big unseen planner pulling the strings.
