big brother

1. An older brother.
2. A man who assumes the role of an older brother, as by providing guidance or protection.
3. 1. also Big Brother An omnipresent, seemingly benevolent figure representing the oppressive control over individual lives exerted by an authoritarian government.
   2. A state, organization, or leader regarded in this manner.

[Sense 3, after Big Brother, a character in the novel 1984 by George Orwell.]

noun

An absolute ruler, especially one who is harsh and oppressive: despot, dictator, führer, man on horseback, oppressor, strongman, totalitarian, tyrant. See over/under.

In George Orwell's 1984, the embodiment of state power is ‘Big Brother’, a middle-aged man pictured almost everywhere and said to be always watching the people. Big Brother was, thus, a powerful, literary image of totalitarianism. For Orwell, he combined elements of Hitler and Stalin, but perhaps also Lord Reith (the first chairman of the BBC), God, and Winston Churchill. The image of Big Brother has entered political language and propaganda and is used to connote the all-embracing power of the state.

— Lincoln Allison

The noun has one meaning:

Meaning #1: an authoritarian leader