Definition of Big Brother noun from the Oxford Advanced American Dictionary

 

Big Brother

 noun
noun
 
[singular]
 
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a leader, a person in authority, or a government that tries to control people's behavior and thoughts, but pretends to act for their benefit From George Orwell's novel Nineteen Eighty-Four, in which the leader of the government, Big Brother, had total control over the people. The slogan “Big Brother is watching you” reminded people that he knew everything they did.
See the Oxford Advanced Learner's Dictionary entry: Big Brother