sit in
phrasal verb with sit verb uk/sɪt/ us/sɪt/ sitting | sat
(COMPLAIN)
（在公共建筑物内）静坐示威
to go as a group into a public building and refuse to leave or to allow normal activities to continue there until a situation that you are complaining about is changed
See also: sit-in
(BE PRESENT)
列席;旁听
to be present in a meeting or class, watching it but not taking part in it
There will be a school inspector sitting in on your class this morning. 今天上午将有一名督学来听你的课。