Where as a demonstration against something men and women occupy a room in a building and have an orgy that is public sex that only stops when there demands are met. They often chain themselves to something to make it hard for the police ect. to drag them off.
noun: a large gathering of people that fornicate in a public setting as a sign of protest.
When Abbie Hoffman, a popular activist of the late sixties, decided to form a mass protest at the 1968 Democratic Convention in Chicago, one of his ideas to protest American involvement in the Vietnam War was to get thousands of yippie and SDS protesters to fornicate on the public grounds of Grant Park (which was one of the larger and popular parks along the lakeside)- thus, a fuck-in.
Similar to phrases like oh my god or jesus christ, fuck-in-a-bucket is meant to be exlamatory or an adjective to describe someone particularly insane or eccentric.