To brainwash someone is to push a belief into their head that is not true, and to keep pushing until they stop fighting it. The methods are ugly: force, fear, mixing them up so they cannot think straight, or saying the same thing again and again for a long time, until the old thoughts feel washed out. The word is strong. News, history, and films use it for prisons, cults, and war. In loose talk people also say it about adverts, social media, or a school that only tells one story — they mean “this message is hitting me so often I cannot think”, not that someone locked them in a room. That loose use is common, but it is weaker than the original. It is a verb: they brainwashed him; she is trying to brainwash the group. Though actually, English did not invent the picture. It copied Chinese 洗腦 — wash the brain — and American soldiers brought the English wording into common use during the Korean War.