sit in on (something)(redirected from sit in on)
sit in on (something)
To attend something as an informal spectator, visitor, or participant. The boss wants to sit in on the production meeting to make sure everything is going smoothly. The professor invited me to sit in on some of his lectures, even though I wasn't enrolled in the college.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.