What does it mean: Self-own
Self-own means a self-inflicted defeat, and a thorough defeat at that.
Flat Earthers are people who believe the world is flat. They don't believe the planet we inhabit is round, i.e., ball-shaped. They believe it's flat, like a plate.
In our example, some Flat Earthers make that point and, apparently, while making an argument to prove that the Earth is flat in a documentary film, inadvertently prove that the Earth is, indeed, round.
Hence, this self-undoing is dubbed a self-own – in fact, the “worst self-own in documentary history.”
Literally, this means you beat and own yourself. In colloquialism in parts of America, if you defeat your opponent badly in a game or debate or something, you are said to “own” your opponent. This expression probably has to do with America's history of slavery. If you own slaves, you have total and absolute control of their lives. Hence, if you own your opponent, you dominate him. You have them under your thumb. They're completely at your mercy.
From this comes the expression, self-own. It means self defeat, a self-inflicted wound. Still thinking about the World Cup, which concluded in a spectacular final between France and Argentina with Lionel Messi winning the title for the first time, I have a soccer analogy for your here. In the game of soccer, an own goal is the ultimate self-own.
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