1.(at home)待在里面；待在家里
She sat in all day waiting for him to come.她整天待在家里等他来。
2.to sit in for sb
(replace sb (in role))代替某人
I had the privilege of sitting in for Kerby Anderson.我享有替代科比·安德森的优先权。
3.(as protest)静坐抗议
The demonstrators sat in in the director's office.示威者在主管办公室里静坐抗议。