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sit in
sit in
(for someone) to act as a substitute for someone. (Usually involves actual sitting, such as at a meeting.) I am not a regular member of this committee. I am sitting in for Larry Smith. Do you mind if I sit in? My representative can't be here.
sit in (on something)
to attend something as a visitor; to act as a temporary participant in something. Do you mind if I sit in on your discussion? Please do sit in.
sit in (for somebody)
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?
sit in
1. Attend or take part as a visitor, as in My son's jazz group asked me to sit in tonight. It is often put as sit in on, as in They asked me to sit in on their poker game. [Mid-1800s]
2. Take part in a sit-in, that is, an organized protest in which seated participants refuse to move. For example, The students threatened to sit in unless the dean was reinstated. [c. 1940]
3. sit in on. Visit or observe, as in I'm sitting in on his class, but not for credit. [Early 1900s]
4. sit in for. Substitute for a regular member of a group, as in I'm just sitting in for Harold, who couldn't make it.
sit in
v.1. To be present or participate as a visitor at a discussion or music session: The professor allowed me to sit in on one of her lectures. A guitarist will be sitting in with the band today.
2. To take part in a sit-in or similar organized protest: The protesters sat in at the construction site.
3. sit in for To act as a substitute: She will be sitting in for the usual news anchor, who is away on vacation.