“Brainwashed” is a common slang term.  Basically, if someone is “brainwashed” it means they believe some theory or idea without really understanding it or giving it much thought.  This term is always used in a negative way.

Here are some common native example sentences:

He believes absolutely everything that is on the news.  He’s so easy to brainwash.

The education system in that country brainwashes all the young kids to think a certain way.

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