to take part in a sit-in protest, remain at home, or join an activity informally
to stay inside, join in by sitting there, or protest by refusing to leave
3 meanings, ordered from most common to least. Color-coded by CEFR level.
to take part in a protest by staying seated in a place and refusing to leave
"The students sat in outside the administration office for two days."
We had students from all over the city sitting in at lunch counters.
— John Lewis, interview reflections on the Civil Rights Movement
to stay at home instead of going out
"It's raining, so let's just sit in tonight and watch a movie."
to join a meeting, class, or musical performance informally or temporarily
"A local guitarist sat in during the last two songs."
Eric Clapton sat in with the band during the encore.
— Common music journalism wording; exact source not recalled
to sit inside a place
to stay inside, join in by sitting there, or protest by refusing to leave
The protest sense is strongly associated with political action. In British English, 'sit in' can also mean stay at home rather than go out.
Natural word combinations native speakers use most often.
The five tense forms you'll use most often.
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