sit in for somebody phrasal verb (see also sit)
to do a job, go to a meeting etc instead of the person who usually does it:
This is Alan James sitting in for Suzy Williams on the mid-morning show.

[TahlilGaran] Dictionary of Contemporary English ▲

sit in for
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 for
to take someone else's place in some activity
I asked my friend to sit in for me at my volunteer job at the community center.

[TahlilGaran] English Idioms Dictionary ▲

sit in for someone
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 for someone
to take someone else's place in some activity
I asked my friend to sit in for me at my volunteer job at the community center.

[TahlilGaran] English Idioms Dictionary ▲