-in²

A gathering of people having a common purpose, typically as a form of protest.

English in.

The first examples, sit-ins, were strikes in the US in the 1930s, in which buildings were occupied as a form of protest; the sense was extended to Black protests in the early 1960s against racial segregation. The ending became very common in the mid 1960s (sleep-in, love-in, pray-in, bed-in, teach-in) but is now mainly historical. The form is modelled on words like lie-in, stand-in, drive-in, plug-in, and shoo-in, all of which predate the 1930s.