1
general::
Phrase(s): 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., Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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.
American Heritage Idioms