Function: intransitive verb
Date: 1868
1 : to take part in or be present at a session of music or discussion as a visitor ― often used with on <invited to sit in on a rehearsal>
2 : to participate in a sit-in
SIT IN
Meaning of SIT IN in English
Merriam Webster Collegiate English Dictionary. Merriam Webster - Энциклопедический словарь английского языка. 2012