sit in

1. 1.

   attend as a visitor

   “Can I sit in on your Intermediate Hittite class?”

   Broaderattend
2. 2.

   participate in an act of civil disobedience

   Broaderdisobey

Questions about sit in

What does sit in mean?

sit in means "attend as a visitor". That is the primary sense; the word has 2 in total, listed in full on this page.

Is sit in a common word?

sit in is banded "Uncommon" — turns up in careful writing rather than conversation. The band comes from how often the word appears in a large corpus of everyday English, not from an editor's impression.