sit in phrasal verb (see also sit)
to be present at a meeting but not take an active part in it
sit in on
Would you like to sit in on some of my interviews?

[TahlilGaran] Dictionary of Contemporary English ▲

ˈsit-in noun [countable]
a type of protest in which people refuse to leave the place where they work or study until their demands are considered or agreed to
hold/stage a sit-in
Several thousand students staged sit-ins and protest marches.

[TahlilGaran] Dictionary of Contemporary English ▲

sit in
sit in (for (someone))
to do someone's job or fulfill a responsibility for them.
The vice president will sit in for the president at today's meeting.

Their drummer was sick so they asked if I could sit in.

sit in (on (something))
to go to a group event without being officially involved.
Do you mind if I sit in on your class?

[TahlilGaran] English Idioms Dictionary ▲

sit in
v.
1. To be a member; participate.
We're having a conference and we'd like you to sit in. also sit in on : To be a member of; participate in.
We want you to sit in on the meeting.
2. To attend but not participate. Often used with "on".
Our teacher was invited to sit in on the conference.
Compare: LOOK ON.

[TahlilGaran] English Idioms Dictionary ▲