1. A form of civil disobedience or nonviolent protest popular during the 1960s. A huge group of people would come to a public place such as a university administration building and simply sit down and refuse to move. Whole buildings would become clogged by people filling the hallways. Usually an army of police were necessary to carry off the protesters one by one.

2. In musical performance, to "sit in" is to informally join an existing ensemble for one song or even a whole performance. This is common in jazz music, where most musicians know a standard repertoire of jazz songs and so ensemble members are totally interchangeable. Many jazz masters started their careers by sitting in with club bands; the band leader or an audience member could then hear them play and hire them.

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